Justin Read

Professor Justin Read


Early Career Researcher Lead and Chair of Astrophysics
+44 (0)1483 689320
06 BB 03

About

Areas of specialism

Astrophysics; Computational Science; Dark Matter; Gravitational Dynamics; Fluid Dynamics; Cosmology; Galaxy Formation

University roles and responsibilities

  • Early Career Researcher Lead

    Previous roles

    01 October 2022 - 01 October 2023
    Interim Head of the School of Mathematics & Physics
    University of Surrey
    01 October 2018 - 01 October 2022
    Head of Physics
    University of Surrey

    Research

    Research interests

    Teaching

    Publications

    Martin P Rey, Andrew Pontzen, Oscar Agertz, Matthew DA Orkney, Justin I Read, Amélie Saintonge, Stacy Y Kim, Payel Das (2022)EDGE: What shapes the relationship between Hi and stellar observables in faint dwarf galaxies?, In: Monthly Notices of the Royal Astronomical Society511(4)pp. 5672-5681 Oxford University Press (OUP)

    We show how the interplay between feedback and mass-growth histories introduces scatter in the relationship between stellar and neutral gas properties of field faint dwarf galaxies (⁠M⋆⪅106M⊙⁠). Across a suite of cosmological, high-resolution zoomed simulations, we find that dwarf galaxies of stellar masses 105≤M⋆≤106M⊙ are bimodal in their cold gas content, being either H i-rich or H i-deficient. This bimodality is generated through the coupling between (i) the modulation of H i contents by the background of ultraviolet radiation (UVB) at late times and (ii) the significant scatter in the stellar-to-halo mass relationship induced by reionization. Furthermore, our H i-rich dwarfs exhibit disturbed and time-variable neutral gas distributions primarily due to stellar feedback. Over the last four billion years, we observe order-of-magnitude changes around the median MHI⁠, factor-of-a-few variations in H i spatial extents, and spatial offsets between H i and stellar components regularly exceeding the galaxies’ optical sizes. Time variability introduces further scatter in the M⋆−MHI relation and affects a galaxy’s detectability in H i at any given time. These effects will need to be accounted for when interpreting observations of the population of faint, H i-bearing dwarfs by the combination of optical and radio wide, deep surveys.

    Gandhali D Joshi, Andrew Pontzen, Oscar Agertz, Martin P Rey, Justin Read, Florent Renaud VINTERGATAN-GM: How do mergers affect the satellite populations of MW-like galaxies?

    We investigate the impact of a galaxy's merger history on its system of satellites using the new \textsc{vintergatan-gm} suite of zoom-in hydrodynamical simulations of Milky Way-mass systems. The suite simulates five realizations of the same halo with targeted `genetic modifications' (GMs) of a $z \approx 2$ merger, but resulting in the same halo mass at $z=0$. We find that differences in the satellite stellar mass functions last for 2.25-4.25 Gyr after the $z \approx 2$ merger; specifically, the haloes that have undergone smaller mergers host up to 60% more satellites than those of the larger merger scenarios. However, by $z=0$ these differences in the satellite stellar mass functions have been erased. The differences in satellite numbers seen soon after the mergers are driven by several factors, including the timings of major mergers, the masses and satellite populations of the central and merging systems, and the subsequent extended history of minor mergers. The results persist when measured at fixed central stellar mass rather than fixed time, implying that a host's recent merger history can be a significant source of scatter when reconstructing its dynamical properties from its satellite population.

    Daniel Vaz, Jarle Brinchmann, Sebastiaan L Zoutendijk, Leindert A Boogaard, Sebastian Kamann, Justin I Read, Martin M Roth, Peter M Weilbacher, Matthias Steinmetz The MUSE-Faint survey. IV. Dissecting Leo T, a gas-rich relic with recent star formation

    Leo T ($M_V = -8.0$) is both the faintest and the least massive galaxy known to contain neutral gas and to display signs of recent star formation. We analyse photometry and stellar spectra to identify member stars and to better understand the overall dynamics and stellar content of the galaxy and to compare the properties of its young and old stars. We use data from the Multi Unit Spectroscopic Explorer (MUSE) on the VLT. We supplement this information with spectroscopic data from the literature and with Hubble Space Telescope (HST) photometry. Our analysis reveals two distinct populations of stars in Leo T. The first population, with an age of $\lesssim 500~\mathrm{Myr}$, includes three emission-line Be stars comprising 15% of the total number of young stars. The second population of stars is much older, with ages ranging from $>5~\mathrm{Gyr}$ to as high as $10~\mathrm{Gyr}$. We combine MUSE data with literature data to obtain an overall velocity dispersion of $\sigma_{v} = 7.07^{+1.29}_{-1.12}~\mathrm{km\ s^{-1}}$ for Leo T. When we divide the sample of stars into young and old populations, we find that they have distinct kinematics. Specifically, the young population has a velocity dispersion of $2.31^{+2.68}_{-1.65}\,\mathrm{km\ s^{-1}}$, contrasting with that of the old population, of $8.14^{+1.66}_{-1.38}\,\mathrm{km\ s^{-1}}$. The fact that the kinematics of the cold neutral gas is in good agreement with the kinematics of the young population suggests that the recent star formation in Leo T is linked with the cold neutral gas. We assess the existence of extended emission-line regions and find none to a surface brightness limit of~$< 1\times 10^{-20}\,\mathrm{erg}\,\mathrm{s}^{-1}\,\mathrm{cm}^{-2}~\mathrm{arcsec}^{-2}$ which corresponds to an upper limit on star formation of $\sim 10^{-11}~\mathrm{M_\odot~yr^{-1}~pc^{-2}}$, implying that the star formation in Leo T has ended.

    Matthew D. A Orkney, Ethan Taylor, Justin I Read, Martin P Rey, Andrew Pontzen, Oscar Agertz, Stacy Y Kim, Maxime Delorme EDGE: The shape of dark matter haloes in the faintest galaxies

    Purely collisionless Dark Matter Only (DMO) structure formation simulations predict that Dark Matter (DM) haloes are typically prolate in their centres and spheroidal towards their outskirts. The addition of gas cooling transforms the central DM shape to be rounder and more oblate. It is not clear, however, whether such shape transformations occur in `ultra-faint' dwarfs, which have extremely low baryon fractions. We present the first study of the shape and velocity anisotropy of ultra-faint dwarf galaxies that have gas mass fractions of $f_{\rm gas}(r

    Michele De Leo, Justin I Read, Noelia E. D Noel, Denis Erkal, Pol Massana, Ricardo Carrera Surviving the Waves: evidence for a Dark Matter cusp in the tidally disrupting Small Magellanic Cloud

    We use spectroscopic data for ${\sim}6,000$ Red Giant Branch (RGB) stars in the Small Magellanic Cloud (SMC), together with proper motion data from \textit{Gaia} Early Data Release 3 (EDR3), to build a mass model of the SMC. We test our Jeans mass modelling method (\textsc{Binulator}+\textsc{GravSphere}) on mock data for an SMC-like dwarf undergoing severe tidal disruption, showing that we are able to successfully remove tidally unbound interlopers, recovering the Dark Matter density and stellar velocity anisotropy profiles within our 95\% confidence intervals. We then apply our method to real SMC data, finding that the stars of the cleaned sample are isotropic at all radii (at 95\% confidence), and that the inner Dark Matter density profile is dense, $\rho_{\rm DM}(150\,{\rm pc}) = 2.81_{-1.07}^{+0.72}\times 10^8 M_{\odot} \rm kpc^{-3} $, consistent with a $\Lambda$ Cold Dark Matter ($\Lambda$CDM) cusp at least down to 400\,pc from the SMC's centre. Our model gives a new estimate of the SMC's total mass within 3\,kpc ($M_{\rm tot} \leq 3\,{\rm kpc})$ of $2.34\pm0.46 \times 10^9 M_{\odot}$. We also derive an astrophysical \textquote{$J$-factor} of $19.22\pm0.14$\, GeV$^2$\,cm$^{-5}$ and a \textquote{$D$-factor} of $18.80\pm0.03$\, GeV$^2$\,cm$^{-5}$, making the SMC a promising target for Dark Matter annihilation and decay searches. Finally, we combine our findings with literature measurements to test models in which Dark Matter is \textquote{heated up} by baryonic effects. We find good qualitative agreement with the Di Cintio et al. 2014 model but we deviate from the Lazar et al. 2020 model at high $M_*/M_{200} > 10^{-2}$. We provide a new, analytic, density profile that reproduces Dark Matter heating behaviour over the range $10^{-5} < M_*/M_{200} < 10^{-1}$.

    Alex Goater, Justin I Read, Noelia E. D Noël, Matthew D. A Orkney, Stacy Y Kim, Martin P Rey, Eric P Andersson, Oscar Agertz, Andrew Pontzen, Roberta Vieliute, Dhairya Kataria, Kiah Jeneway EDGE: The direct link between mass growth history and the extended stellar haloes of the faintest dwarf galaxies

    Ultra-faint dwarf galaxies (UFDs) are commonly found in close proximity to the Milky Way and other massive spiral galaxies. As such, their projected stellar ellipticity and extended light distributions are often thought to owe to tidal forces. In this paper, we study the projected stellar ellipticities and faint stellar outskirts of tidally isolated ultra-faints drawn from the 'Engineering Dwarfs at Galaxy Formation's Edge' (EDGE) cosmological simulation suite. Despite their tidal isolation, our simulated dwarfs exhibit a wide range of projected ellipticities ($0.03 < \varepsilon < 0.85$), with many possessing anisotropic extended stellar haloes that mimic tidal tails, but owe instead to late-time accretion of lower mass companions. Furthermore, we find a strong causal relationship between ellipticity and formation time of an UFD, which is robust to a wide variation in the feedback model. We show that the distribution of projected ellipticities in our suite of simulated EDGE dwarfs matches well with that of 21 Local Group dwarf galaxies. Given the ellipticity in EDGE arises from an ex-situ accretion origin, the agreement in shape indicates the ellipticities of some observed dwarfs may also originate from a similar non-tidal scenario. The orbital parameters of these observed dwarfs further support that they are not currently tidally disrupting. If the baryonic content in these galaxies is still tidally intact, then the same may be true for their dark matter content, making these galaxies in our Local Group pristine laboratories for testing dark matter and galaxy formation models.

    Mariana P Júlio, Jarle Brinchmann, Sebastiaan L Zoutendijk, Justin I Read, Daniel Vaz, Sebastian Kamann, Davor Krajnović, Leindert A Boogaard, Matthias Steinmetz, Nicolas Bouché The MUSE-Faint survey. V. Constraining Scalar Field Dark Matter with Antlia B

    Aims. We use stellar line-of-sight velocities of Antlia B, a faint dwarf galaxy in the NGC 3109 association, to derive constraints on the fundamental properties of scalar field dark matter originally proposed to solve the small-scale problems faced by cold dark matter models. Methods. We use the first spectroscopic observations of Antlia B, a distant (d $\sim$ 1.35 Mpc) faint dwarf ($M_\text{V} = -9.7$, M$_\star \sim 8\times10^5$M$_\odot$), from MUSE-Faint - a survey of ultra-faint dwarfs with the Multi Unit Spectroscopic Explorer. Through measurement of line-of-sight velocities for stars in the $1'\times 1'$ field-of-view, we identify 127 stars as members of Antlia B, allowing us to model its dark matter density profile with the Jeans modelling code GravSphere. We implement a model for scalar field dark matter into GravSphere and use this to place constraints on the self-coupling strength of this model. Results. We find a virial mass of ${M_{200} \approx 1.66^{+2.51}_{-0.92}\times 10^9}$ M$_\odot$ and a concentration parameter of ${c_{200}\approx 17.38^{+6.06}_{-4.20}}$ for Antlia B. These results are consistent with the mass-concentration relations in the literature. We constrain the characteristic length scale of the repulsive self-interaction $R_{\text{TF}}$ of the scalar field dark matter model to $R_{\text{TF}} \lesssim 180$ pc (68% confidence level), which translates to a self-coupling strength of $\frac{g}{m^2c^4}\lesssim 5.2 \times 10^{-20}$ eV$^{-1}$cm$^3$. The constraint on the characteristic length scale of the repulsive self-interaction is inconsistent with the value required to match the observations of cores of dwarf galaxies in the Local Group, suggesting that the cored density profiles of those galaxies are not caused by scalar field dark matter.

    Pengfei Li, Yong Tian, Mariana P Júlio, Marcel S Pawlowski, Federico Lelli, Stacy S McGaugh, James M Schombert, Justin I Read, Po-Chieh Yu, Chung-Ming Ko Measuring galaxy cluster mass profiles into the low acceleration regime with galaxy kinematics

    We probe the dynamical mass profiles of 10 galaxy clusters from the HIghest X-ray FLUx Galaxy Cluster Sample (HIFLUGCS) using galaxy kinematics. We numerically solve the spherical Jeans equation, and parameterize the dynamical mass profile and the galaxy velocity anisotropy profile using two general functions to ensure that our results are not biased towards any specific model. The mass-velocity anisotropy degeneracy is ameliorated by using two "virial shape parameters" that depend on the fourth moment of velocity distribution. The resulting velocity anisotropy estimates consistently show a nearly isotropic distribution in the inner regions, with an increasing radial anisotropy towards large radii. We compare our derived dynamical masses with those calculated from X-ray gas data assuming hydrostatic equilibrium, finding that massive and rich relaxed clusters generally present consistent mass measurements, while unrelaxed or low-richness clusters have systematically larger total mass than hydrostatic mass by an average of 50\%. This might help alleviate current tensions in the measurement of $\sigma_8$, but it also leads to cluster baryon fractions below the cosmic value. Finally, our approach probes accelerations as low as $10^{-11}$ m s$^{-2}$, comparable to the outskirts of individual late-type galaxies. We confirm that galaxy clusters deviate from the radial acceleration relation defined by galaxies.

    Matthew D A Orkney, Ethan Taylor, Justin I Read, Martin P Rey, A Pontzen, Oscar Agertz, Stacy Y Kim, Maxime Delorme (2023)EDGE: The shape of dark matter haloes in the faintest galaxies, In: Monthly Notices of the Royal Astronomical Societypp. 3516-3532 Oxford University Press

    Collisionless Dark Matter Only (DMO) structure formation simulations predict that Dark Matter(DM) haloes are prolate in their centres and triaxial towards their outskirts. The addition of gas condensation transforms the central Dark Matter(DM) shape to be rounder and more oblate. It is not clear, however, whether such shape transformations occur in ‘ultra-faint’ dwarfs, which have extremely low baryon fractions. We present the first study of the shape and velocity anisotropy of ultra-faint dwarf galaxies that have gas mass fractions of fgas(r < Rhalf) < 0.06. These dwarfs are drawn from the Engineering Dwarfs at Galaxy formation's Edge (EDGE) project, using high resolution simulations that allow us to resolve Dark Matter(DM) halo shapes within the half light radius (∼100 pc). We show that gas-poor ultra-faints (M200c ≤ 1.5 × 109 M⊙; fgas < 10−5) retain their pristine prolate Dark Matter(DM) halo shape even when gas, star formation and feedback are included. This could provide a new and robust test of Dark Matter(DM) models. By contrast, gas-rich ultra-faints (M200c > 3 × 109 M⊙; fgas > 10−4) become rounder and more oblate within ∼10 half light radii. Finally, we find that most of our simulated dwarfs have significant radial velocity anisotropy that rises to $\tilde{\beta } > 0.5$ at R ≳ 3Rhalf. The one exception is a dwarf that forms a rotating gas/stellar disc because of a planar, major merger. Such strong anisotropy should be taken into account when building mass models of gas-poor ultra-faints.

    Alex Goater, Justin I Read, Noelia E D Noël, Matthew D A Orkney, Stacy Y Kim, Martin P Rey, Eric P Andersson, Oscar Agertz, Andrew Pontzen, Roberta Vieliute, Dhairya Kataria, Kiah Jeneway (2023)EDGE: The direct link between mass growth history and the extended stellar haloes of the faintest dwarf galaxies, In: Monthly notices of the Royal Astronomical Society527(2)pp. 2403-2412 Oxford University Press

    Ultra-faint dwarf galaxies (UFDs) are commonly found in close proximity to the Milky Way and other massive spiral galaxies. As such, their projected stellar ellipticity and extended light distributions are often thought to owe to tidal forces. In this paper, we study the projected stellar ellipticities and faint stellar outskirts of tidally isolated ultra-faints drawn from the ‘Engineering Dwarfs at Galaxy Formation’s Edge’ (EDGE) cosmological simulation suite. Despite their tidal isolation, our simulated dwarfs exhibit a wide range of projected ellipticities (0.03 < ε < 0.85), with many possessing anisotropic extended stellar haloes that mimic tidal tails, but owe instead to late-time accretion of lower mass companions. Furthermore, we find a strong causal relationship between ellipticity and formation time of a UFD, which is robust to a wide variation in the feedback model. We show that the distribution of projected ellipticities in our suite of simulated EDGE dwarfs matches well with a sample of 19 Local Group dwarf galaxies and a sample of 11 isolated dwarf galaxies. Given ellipticity in EDGE arises from an ex-situ accretion origin, the agreement in shape indicates the ellipticities of some observed dwarfs may also originate from a non-tidal scenario. The orbital parameters of these observed dwarfs further support that they are not currently tidally disrupting. If the baryonic content in these galaxies is still tidally intact, then the same may be true for their dark matter content, making these galaxies in our Local Group pristine laboratories for testing dark matter and galaxy formation models.

    Mariana Júlio, Jarle Brinchmann, Sebastiaan Zoutendijk, Justin Read, Daniel Vaz, Sebastian Kamann, Davor Krajnović, Leindert Boogaard, Matthias Steinmetz, Nicolas Bouché (2023)The MUSE-Faint survey, In: Astronomy and astrophysics (Berlin)678 EDP Sciences

    Aims. We use the stellar line-of-sight velocities of Antlia B (Ant B), a faint dwarf galaxy in the NGC 3109 association, to derive constraints on the fundamental properties of scalar field dark matter (SFDM), which was originally proposed to solve the small-scale problems faced by cold dark matter models. Methods. We used the first spectroscopic observations of Ant B, a distant (d ∼ 1.35 Mpc) faint dwarf (MV = −9.7, M⋆ ∼ 8 × 105 M⊙), from MUSE-Faint, a survey of ultra-faint dwarfs conducted using the Multi Unit Spectroscopic Explorer. By measuring the line-of-sight velocities of stars in the 1′×1′ field of view, we identified 127 stars as members of Ant B, which enabled us to model its dark matter density profile with the Jeans modelling code GRAVSPHERE. We implemented a model for SFDM into GRAVSPHERE and used this to place constraints on the self-coupling strength of this model. Results. We find a virial mass of M200 ≈ 1.66−0.92+2.51 × 109 M⊙ and a concentration parameter of c200 ≈ 17.38−4.20+6.06 for Ant B. These results are consistent with the mass-concentration relations in the literature. We constrain the characteristic length scale of the repulsive self-interaction RTF of the SFDM model to RTF ≲ 180 pc (68% confidence level), which translates to a self-coupling strength of g/m2c4 ≲ 5.2 × 10−20 eV−1 cm3. The constraint on the characteristic length scale of the repulsive self-interaction is inconsistent with the value required to match observations of the cores of dwarf galaxies in the Local Group, suggesting that the cored density profiles of those galaxies are not caused by SFDM.

    Yifei Luo, Alexie Leauthaud, Jenny Greene, Song Huang, Erin Kado-Fong, Shany Danieli, Ting S Li, Jiaxuan Li, Diana Blanco, Erik J Wasleske, Joseph Wick, Abby Mintz, Runquan Guan, Annika H. G Peter, Vivienne Baldassare, Alyson Brooks, Arka Banerjee, Joy Bhattacharyya, Zheng Cai, Xinjun Chen, Jim Gunn, Sean D Johnson, Lee S Kelvin, Mingyu Li, Xiaojing Lin, Robert Lupton, Charlie Mace, Gustavo E Medina, Justin Read, Rodrigo Cordova Rosado, Allen Seifert The Merian Survey: Design, Construction, and Characterization of a Filter Set Optimized to Find Dwarf Galaxies and Measure their Dark Matter Halo Properties with Weak Lensing

    The Merian survey is mapping $\sim$ 850 degrees$^2$ of the Hyper Suprime-Cam Strategic Survey Program (HSC-SSP) wide layer with two medium-band filters on the 4-meter Victor M. Blanco telescope at the Cerro Tololo Inter-American Observatory, with the goal of carrying the first high signal-to-noise (S/N) measurements of weak gravitational lensing around dwarf galaxies. This paper presents the design of the Merian filter set: N708 ($\lambda_c = 7080 \unicode{x212B}$, $\Delta\lambda = 275\unicode{x212B}$) and N540 ($\lambda_c = 5400\unicode{x212B}$, $\Delta\lambda = 210\unicode{x212B}$). The central wavelengths and filter widths of N708 and N540 were designed to detect the $\rm H\alpha$ and $\rm [OIII]$ emission lines of galaxies in the mass range $8

    Imran Nasim, Alessia Gualandris, Justin Read, Walter Dehnen, Maxime Delorme, Fabio Antonini Defeating stochasticity: coalescence timescales of massive black holes in galaxy mergers

    The coalescence of massive black hole binaries (BHBs) in galactic mergers is the primary source of gravitational waves (GWs) at low frequencies. Current estimates of GW detection rates for the Laser Interferometer Space Antenna and the Pulsar Timing Array vary by three orders of magnitude. To understand this variation, we simulate the merger of equal-mass, eccentric, galaxy pairs with central massive black holes and shallow inner density cusps. We model the formation and hardening of a central BHB using the Fast Multiple Method as a force solver, which features a $O(N)$ scaling with the number $N$ of particles and obtains results equivalent to direct-summation simulations. At $N \sim 5\times 10^5$, typical for contemporary studies, the eccentricity of the BHBs can vary significantly for different random realisations of the same initial condition, resulting in a substantial variation of the merger timescale. This scatter owes to the stochasticity of stellar encounters with the BHB and decreases with increasing $N$. We estimate that $N \sim 10^7$ within the stellar half-light radius suffices to reduce the scatter in the merger timescale to $\sim 10$\%. Our results suggest that at least some of the uncertainty in low-frequency GW rates owes to insufficient numerical resolution.

    Daisuke Kawata, Hajime Kawahara, Naoteru Gouda, Nathan J Secrest, Ryouhei Kano, Hirokazu Kataza, Naoki Isobe, Ryou Ohsawa, Fumihiko Usui, Yoshiyuki Yamada, Alister W Graham, Alex R Pettitt, Hideki Asada, Junichi Baba, Kenji Bekki, Bryan N Dorland, Michiko Fujii, Akihiko Fukui, Kohei Hattori, Teruyuki Hirano, Takafumi Kamizuka, Shingo Kashima, Norita Kawanaka, Yui Kawashima, Sergei A Klioner, Takanori Kodama, Naoki Koshimoto, Takayuki Kotani, Masayuki Kuzuhara, Stephen E Levine, Steven R Majewski, Kento Masuda, Noriyuki Matsunaga, Kohei Miyakawa, Makoko Miyoshi, Kumiko Morihana, Ryoichi Nishi, Yuta Notsu, Masashi Omiya, Jason Sanders, Ataru Tanikawa, Masahiro Tsujimoto, Taihei Yano, Masataka Aizawa, Ko Arimatsu, Michael Biermann, Celine Boehm, Masashi Chiba, Victor P Debattista, Ortwin Gerhard, Masayuki Hirabayashi, David Hobbs, Bungo Ikenoue, Hideyuki Izumiura, Carme Jordi, Naoki Kohara, Wolfgang Löffler, Xavier Luri, Ichiro Mase, Andrea Miglio, Kazuhisa Mitsuda, Trent Newswander, Shogo Nishiyama, Yoshiyuki Obuchi, Takafumi Ootsubo, Masami Ouchi, Masanobu Ozaki, Michael Perryman, Timo Prusti, Pau Ramos, Justin I Read, R. Michael Rich, Ralph Schönrich, Minori Shikauchi, Risa Shimizu, Yoshinori Suematsu, Shotaro Tada, Aoi Takahashi, Takayuki Tatekawa, Daisuke Tatsumi, Takuji Tsujimoto, Toshihiro Tsuzuki, Seitaro Urakawa, Fumihiro Uraguchi, Shin Utsunomiya, Vincent Van Eylen, Floor van Leeuwen, Takehiko Wada, Nicholas A Walton JASMINE: Near-Infrared Astrometry and Time Series Photometry Science

    Japan Astrometry Satellite Mission for INfrared Exploration (JASMINE) is a planned M-class science space mission by the Institute of Space and Astronautical Science, the Japan Aerospace Exploration Agency. JASMINE has two main science goals. One is Galactic archaeology with Galactic Center Survey, which aims to reveal the Milky Way's central core structure and formation history from Gaia-level (~25 $\mu$as) astrometry in the Near-Infrared (NIR) Hw-band (1.0-1.6 $\mu$m). The other is the Exoplanet Survey, which aims to discover transiting Earth-like exoplanets in the habitable zone from the NIR time-series photometry of M dwarfs, when the Galactic center is not accessible. We introduce the mission, review many science objectives and present the instrument concept. JASMINE will be the first dedicated NIR astrometry space mission and provide precise astrometric information of the stars in the Galactic center, taking advantage of the significantly lower extinction in the NIR band. The precise astrometry is obtained by taking many short-exposure images. Hence, the JASMINE Galactic center survey data will be valuable for studies of exoplanet transits, asteroseismology, variable stars and microlensing studies, including discovery of (intermediate mass) black holes. We highlight a swath of such potential science, and also describe synergies with other missions.

    Martin P Rey, Matthew D. A Orkney, Justin I Read, Payel Das, Oscar Agertz, Andrew Pontzen, Anastasia A Ponomareva, Stacy Y Kim, William McClymont EDGE -- Dark matter or astrophysics? Clear prospects to break dark matter heating degeneracies with HI rotation in faint dwarf galaxies

    Low-mass dwarf galaxies are expected to showcase pristine `cuspy' inner dark matter density profiles compared to their stellar sizes, as they form too few stars to significantly drive dark matter heating through supernovae-driven outflows. Here, we study such simulated faint systems ($10^4 \leq M_{\star} \leq 2\times 10^6 \, M_\mathrm{\odot}$) drawn from high-resolution (3 pc) cosmological simulations from the `Engineering Dwarf Galaxies at the Edge of galaxy formation' (EDGE) project. We confirm that these objects have steep and rising inner dark matter density profiles at $z=0$, little affected by galaxy formation effects. But five dwarf galaxies from the suite showcase a detectable HI reservoir ($M_{\mathrm{HI}}\approx 10^{5}-10^{6} \, M_\mathrm{\odot}$), analogous to the observed population of faint, HI-bearing dwarf galaxies. These reservoirs exhibit episodes of ordered rotation, opening windows for rotation curve analysis. Within actively star-forming dwarfs, stellar feedback easily disrupts the tenuous HI discs ($v_{\phi} \approx 10\, \mathrm{km} \, \mathrm{s}^{-1}$), making rotation short-lived ($\ll 150 \, \mathrm{Myr}$) and more challenging to interpret for dark matter inferences. Contrastingly, we highlight a long-lived ($\geq 500 \, \mathrm{Myr}$) and easy-to-interpret HI rotation curve extending to $\approx 2\, r_{1/2, \text{3D}}$ in a quiescent dwarf, that has not formed new stars since $z=4$. This stable gas disc is supported by an oblate dark matter halo shape that drives high angular momentum gas flows. Our results strongly motivate further searches for HI rotation curves in the observed population of HI-bearing low-mass dwarfs, that provide a key regime to disentangle the respective roles of dark matter microphysics and galaxy formation effects in driving dark matter heating.

    Claudia Muni, Andrew Pontzen, Jason L Sanders, Martin P Rey, Justin I Read, Oscar Agertz From particles to orbits: precise dark matter density profiles using dynamical information

    We introduce a new method to calculate dark matter halo density profiles from simulations. Each particle is 'smeared' over its orbit to obtain a dynamical profile that is averaged over a dynamical time, in contrast to the traditional approach of binning particles based on their instantaneous positions. The dynamical and binned profiles are in good agreement, with the dynamical approach showing a significant reduction in Poisson noise in the innermost regions. We find that the inner cusps of the new dynamical profiles continue inward all the way to the softening radius, reproducing the central density profile of higher resolution simulations within the 95$\%$ confidence intervals, for haloes in virial equilibrium. Folding in dynamical information thus provides a new approach to improve the precision of dark matter density profiles at small radii, for minimal computational cost. Our technique makes two key assumptions: that the halo is in equilibrium (phase mixed), and that the potential is spherically symmetric. We discuss why the method is successful despite strong violations of spherical symmetry in the centres of haloes, and explore how substructures disturb equilibrium at large radii.

    P Saha, J I Read (2009)The cluster lens ACO 1703: redshift contrast and the inner profile Institute of Physics Publishing

    ACO 1703 is a cluster recently found to have a variety of strongly lensed objects: there is a quintuply imaged system at z = 0.888 and several other lensed objects from z = 2.2 to 3.0 (the cluster itself is at z = 0.28). It is not difficult to model the lens, as previous work has already done. However, lens models are generically nonunique. We generate ensembles of models to explore the nonuniqueness. When the full range of source redshifts is included, all models are close to ρ vprop r –1 out to 200 kpc. But if the quint is omitted, both shallower and steeper models (e.g., ρ vprop r –2) are possible. The reason is that the redshift contrast between the quint and the other sources gives a good measurement of the enclosed mass at two different radii, thus providing a good estimate of the mass profile in between. This result supports universal profiles and explains why single-model approaches can give conflicting results. The mass map itself is elongated in the northwest-southeast direction, like the galaxy distribution. An overdensity in both mass and light is also apparent to the southeast, which suggests mesostructure.

    Andrew Pontzen, Justin I Read, Romain Teyssier, Fabio Governato, Alessia Gualandris, Nina Roth, Julien Devriendt (2015)Milking the spherical cow - on aspherical dynamics in spherical coordinates Oxford University Press

    Galaxies and the dark matter haloes that host them are not spherically symmetric, yet spherical symmetry is a helpful simplifying approximation for idealized calculations and analysis of observational data. The assumption leads to an exact conservation of angular momentum for every particle, making the dynamics unrealistic. But how much does that inaccuracy matter in practice for analyses of stellar distribution functions, collisionless relaxation, or dark matter core-creation? We provide a general answer to this question for a wide class of aspherical systems; specifically, we consider distribution functions that are `maximally stable', i.e. that do not evolve at first order when external potentials (which arise from baryons, large-scale tidal fields or infalling substructure) are applied. We show that a spherically symmetric analysis of such systems gives rise to the false conclusion that the density of particles in phase space is ergodic (a function of energy alone). Using this idea we are able to demonstrate that: (a) observational analyses that falsely assume spherical symmetry are made more accurate by imposing a strong prior preference for near-isotropic velocity dispersions in the centre of spheroids; (b) numerical simulations that use an idealized spherically symmetric setup can yield misleading results and should be avoided where possible; and (c) triaxial dark matter haloes (formed in collisionless cosmological simulations) nearly attain our maximally stable limit, but their evolution freezes out before reaching it.

    A C Boley, G Lake, J Read, R Teyssier (2009)Globular cluster formation within a cosmological context IOP Publishing

    We place constraints on the formation redshifts for blue globular clusters (BGCs), independent of the details of hydrodynamics and population III star formation. The observed radial distribution of BGCs in the Milky Way Galaxy suggests that they formed in biased dark matter halos at high redshift. As a result, simulations of a ~1 Mpc box up to z ~ 10 must resolve BGC formation in ΛCDM. We find that most halo stars could be produced from destroyed BGCs and other low-mass clusters that formed at high redshift. We present a proof-of-concept simulation that captures the formation of globular-like star clusters.

    H Lux, J I Read, G Lake (2010)Determining orbits for the Milky Way's dwarfs Wiley-Blackwell

    We calculate orbits for the Milky Way dwarf galaxies with proper motions, and compare these to subhalo orbits in a high-resolution cosmological simulation. We use the simulation data to assess how well orbits may be recovered in the face of measurement errors, a time-varying triaxial gravitational potential and satellite-satellite interactions. For present measurement uncertainties, we recover the apocentre ra and pericentre rp to ~40 per cent. With improved data from the Gaia satellite we should be able to recover ra and rp to ~14 per cent, respectively. However, recovering the 3D positions and orbital phase of satellites over several orbits is more challenging. This owes primarily to the non-sphericity of the potential and satellite interactions during group infall. Dynamical friction, satellite mass-loss and the mass evolution of the main halo play a more minor role in the uncertainties. We apply our technique to nine Milky Way dwarfs with observed proper motions. We show that their mean apocentre is lower than the mean of the most massive subhaloes in our cosmological simulation, but consistent with the most massive subhaloes that form before z = 10. This lends further support to the idea that the Milky Way's dwarfs formed before reionization.

    Federico Sembolini, Gustavo Yepes, Frazer R Pearce, Alexander Knebe, Scott T Kay, Chris Power, Weiguang Cui, Alexander M Beck, Stefano Borgani, Claudio Dalla Vecchia, Romeel Davé, Pascal Jahan Elahi, Sean February, Shuiyao Huang, Alex Hobbs, Neal Katz, Erwin Lau, Ian G McCarthy, Guiseppe Murante, Daisuke Nagai, Kaylea Nelson, Richard D A Newton, Valentin Perret, Ewald Puchwein, Justin I Read, Alexandro Saro, Joop Schaye, Romain Teyssier, Robert J Thacker (2016)nIFTy galaxy cluster simulations – I. Dark matter and non-radiative models Oxford University Press

    We have simulated the formation of a galaxy cluster in a Λ cold dark matter universe using 13 different codes modelling only gravity and non-radiative hydrodynamics (RAMSES, ART, AREPO, HYDRA and nine incarnations of GADGET). This range of codes includes particle-based, moving and fixed mesh codes as well as both Eulerian and Lagrangian fluid schemes. The various GADGET implementations span classic and modern smoothed particle hydrodynamics (SPH) schemes. The goal of this comparison is to assess the reliability of cosmological hydrodynamical simulations of clusters in the simplest astrophysically relevant case, that in which the gas is assumed to be non-radiative. We compare images of the cluster at z = 0, global properties such as mass and radial profiles of various dynamical and thermodynamical quantities. The underlying gravitational framework can be aligned very accurately for all the codes allowing a detailed investigation of the differences that develop due to the various gas physics implementations employed. As expected, the mesh-based codes RAMSES, ART and AREPO form extended entropy cores in the gas with rising central gas temperatures. Those codes employing classic SPH schemes show falling entropy profiles all the way into the very centre with correspondingly rising density profiles and central temperature inversions. We show that methods with modern SPH schemes that allow entropy mixing span the range between these two extremes and the latest SPH variants produce gas entropy profiles that are essentially indistinguishable from those obtained with grid-based methods.

    Oscar Agertz, Ben Moore, Joachim Stadel, Doug Potter, Francesco Miniati, Justin Read, Lucio Mayer, Artur Gawryszczak, Andrey Kravtsov, Åke Nordlund, Frazer Pearce, Vicent Quilis, Douglas Rudd, Volker Springel, James Stone, Elizabeth Tasker, Romain Teyssier, James Wadsley, Rolf Walder (2007)Fundamental differences between SPH and grid methods Oxford University Press

    We have carried out a comparison study of hydrodynamical codes by investigating their performance in modelling interacting multiphase fluids. The two commonly used techniques of grid and smoothed particle hydrodynamics (SPH) show striking differences in their ability to model processes that are fundamentally important across many areas of astrophysics. Whilst Eulerian grid based methods are able to resolve and treat important dynamical instabilities, such as Kelvin-Helmholtz or Rayleigh-Taylor, these processes are poorly or not at all resolved by existing SPH techniques. We show that the reason for this is that SPH, at least in its standard implementation, introduces spurious pressure forces on particles in regions where there are steep density gradients. This results in a boundary gap of the size of an SPH smoothing kernel radius over which interactions are severely damped

    S Garbari, J I Read, G Lake (2012)Limits on the local dark matter density EDP Sciences

    We study the systematic problems in determining the local dark matter density ρdm(Rsun) from kinematics of stars in the Solar Neighbourhood, using a simulated Milky Way-like galaxy. We introduce a new unbiased method for recovering ρdm(Rsun) based on the moments of the Jeans equations, combined with a Monte Carlo Markov Chain (MCMC) technique and apply it to real data [1].

    G. Gilmore, S. Randich, C. C. Worley, A. Hourihane, A. Gonneau, G. G. Sacco, J. R. Lewis, L. Magrini, P. Francois, R. D. Jeffries, S. E. Koposov, A. Bragaglia, E. J. Alfaro, C. Allende Prieto, R. Blomme, A. J. Korn, A. C. Lanzafame, E. Pancino, A. Recio-Blanco, R. Smiljanic, S. Van Eck, T. Zwitter, T. Bensby, E. Flaccomio, M. J. Irwin, E. Franciosini, L. Morbidelli, F. Damiani, R. Bonito, E. D. Friel, J. S. Vink, L. Prisinzano, U. Abbas, D. Hatzidimitriou, E. , V Held, C. Jordi, E. Paunzen, A. Spagna, R. J. Jackson, J. Maiz Apellaniz, M. Asplund, P. Bonifacio, S. Feltzing, J. Binney, J. Drew, A. M. N. Ferguson, G. Micela, I Negueruela, T. Prusti, H-W Rix, A. Vallenari, M. Bergemann, A. R. Casey, P. de Laverny, A. Frasca, V Hill, K. Lind, L. Sbordone, S. G. Sousa, V Adibekyan, E. Caffau, S. Daflon, D. K. Feuillet, M. Gebran, J. , I Gonzalez Hernandez, G. Guiglion, A. Herrero, A. Lobel, D. Montes, T. Morel, G. Ruchti, C. Soubiran, H. M. Tabernero, G. Tautvaisiene, G. Traven, M. Valentini, M. Van der Swaelmen, S. Villanova, C. Viscasillas Vazquez, A. Bayo, K. Biazzo, G. Carraro, B. Edvardsson, U. Heiter, P. Jofre, G. Marconi, C. Martayan, T. Masseron, L. Monaco, N. A. Walton, S. Zaggia, V. Aguirre Borsen-Koch, J. Alves, L. Balaguer-Nunez, P. S. Barklem, D. Barrado, M. Bellazzini, S. R. Berlanas, A. S. Binks, A. Bressan, R. Capuzzo-Dolcetta, L. Casagrande, L. Casamiquela, R. S. Collins, V D'Orazi, M. L. L. Dantas, V. P. Debattista, E. Delgado-Mena, P. Di Marcantonio, A. Drazdauskas, N. W. Evans, B. Famaey, M. Franchini, Y. Fremat, X. Fu, D. Geisler, O. Gerhard, E. A. Gonzalez Solares, E. K. Grebel, M. L. Gutierrez Albarran, F. Jimenez-Esteban, Henrik Jönsson, T. Khachaturyants, G. Kordopatis, J. Kos, N. Lagarde, H-G Ludwig, L. Mahy, M. Mapelli, E. Marfil, S. L. Martell, S. Messina, A. Miglio, I Minchev, A. Moitinho, J. Montalban, M. J. P. F. G. Monteiro, C. Morossi, N. Mowlavi, A. Mucciarelli, D. N. A. Murphy, N. Nardetto, S. Ortolani, F. Paletou, J. Palous, J. C. Pickering, A. Quirrenbach, P. Re Fiorentin, J. , I Read, D. Romano, N. Ryde, N. Sanna, W. Santos, G. M. Seabroke, L. Spina, M. Steinmetz, E. Stonkute, E. Sutorius, F. Thevenin, M. Tosi, M. Tsantaki, N. Wright, R. F. G. Wyse, M. Zoccali, J. Zorec, D. B. Zucker (2022)The Gaia-ESO Public Spectroscopic Survey, In: Astronomy and astrophysics (Berlin)666120

    Context. The Gaia-ESO Public Spectroscopic Survey is an ambitious project designed to obtain astrophysical parameters and elemental abundances for 100 000 stars, including large representative samples of the stellar populations in the Galaxy, and a well-defined sample of 60 (plus 20 archive) open clusters. We provide internally consistent results calibrated on benchmark stars and star clusters, extending across a very wide range of abundances and ages. This provides a legacy data set of intrinsic value, and equally a large wide-ranging dataset that is of value for the homogenisation of other and future stellar surveys and Gaia's astrophysical parameters. Aims. This article provides an overview of the survey methodology, the scientific aims, and the implementation, including a description of the data processing for the GIRAFFE spectra. A companion paper introduces the survey results. Methods. Gaia-ESO aspires to quantify both random and systematic contributions to measurement uncertainties. Thus, all available spectroscopic analysis techniques are utilised, each spectrum being analysed by up to several different analysis pipelines, with considerable effort being made to homogenise and calibrate the resulting parameters. We describe here the sequence of activities up to delivery of processed data products to the ESO Science Archive Facility for open use. Results. The Gaia-ESO Survey obtained 202 000 spectra of 115 000 stars using 340 allocated VLT nights between December 2011 and January 2018 from GIRAFFE and UVES. Conclusions. The full consistently reduced final data set of spectra was released through the ESO Science Archive Facility in late 2020, with the full astrophysical parameters sets following in 2022. A companion article reviews the survey implementation, scientific highlights, the open cluster survey, and data products.

    S. Randich, G. Gilmore, L. Magrini, G. G. Sacco, R. J. Jackson, R. D. Jeffries, C. C. Worley, A. Hourihane, A. Gonneau, C. Viscasillas Vazquez, E. Franciosini, J. R. Lewis, E. J. Alfaro, C. Allende Prieto, T. Bensby, R. Blomme, A. Bragaglia, E. Flaccomio, P. Francois, M. J. Irwin, S. E. Koposov, A. J. Korn, A. C. Lanzafame, E. Pancino, A. Recio-Blanco, R. Smiljanic, S. Van Eck, T. Zwitter, M. Asplund, P. Bonifacio, S. Feltzing, J. Binney, J. Drew, A. M. N. Ferguson, G. Micela, I Negueruela, T. Prusti, H-W Rix, A. Vallenari, A. Bayo, M. Bergemann, K. Biazzo, G. Carraro, A. R. Casey, F. Damiani, A. Frasca, U. Heiter, V Hill, P. Jofre, P. de Laverny, K. Lind, G. Marconi, C. Martayan, T. Masseron, L. Monaco, L. Morbidelli, L. Prisinzano, L. Sbordone, S. G. Sousa, S. Zaggia, V Adibekyan, R. Bonito, E. Caffau, S. Daflon, D. K. Feuillet, M. Gebran, J. , I Gonzalez Hernandez, G. Guiglion, A. Herrero, A. Lobel, J. Maiz Apellaniz, D. Montes, T. Morel, C. Soubiran, L. Spina, H. M. Tabernero, G. Traven, M. Valentini, M. Van der Swaelmen, S. Villanova, N. J. Wright, U. Abbas, V. Aguirre Borsen-Koch, J. Alves, L. Balaguer-Nunez, P. S. Barklem, D. Barrado, S. R. Berlanas, A. S. Binks, A. Bressan, R. Capuzzo-Dolcetta, L. Casagrande, L. Casamiquela, R. S. Collins, V D'Orazi, M. L. L. Dantas, V. P. Debattista, E. Delgado-Mena, P. Di Marcantonio, A. Drazdauskas, N. W. Evans, B. Famaey, M. Franchini, Y. Fremat, E. D. Friel, X. Fu, D. Geisler, O. Gerhard, E. A. Gonzalez Solares, E. K. Grebel, M. L. Gutierrez Albarran, D. Hatzidimitriou, E. , V Held, F. Jimenez-Esteban, Henrik Jönsson, C. Jordi, T. Khachaturyants, G. Kordopatis, J. Kos, N. Lagarde, L. Mahy, M. Mapelli, E. Marfil, S. L. Martell, S. Messina, A. Miglio, I Minchev, A. Moitinho, J. Montalban, M. J. P. F. G. Monteiro, C. Morossi, N. Mowlavi, A. Mucciarelli, D. N. A. Murphy, N. Nardetto, S. Ortolani, F. Paletou, J. Palous, E. Paunzen, J. C. Pickering, A. Quirrenbach, P. Re Fiorentin, J. , I Read, D. Romano, N. Ryde, N. Sanna, W. Santos, G. M. Seabroke, A. Spagna, M. Steinmetz, E. Stonkute, E. Sutorius, F. Thevenin, M. Tosi, M. Tsantaki, J. S. Vink, N. Wright, R. F. G. Wyse, M. Zoccali, J. Zorec, D. B. Zucker, N. A. Walton (2022)The Gaia-ESO Public Spectroscopic Survey, In: Astronomy and astrophysics (Berlin)666A121

    Context. In the last 15 years different ground-based spectroscopic surveys have been started (and completed) with the general aim of delivering stellar parameters and elemental abundances for large samples of Galactic stars, complementing Gaia astrometry. Among those surveys, the Gaia-ESO Public Spectroscopic Survey, the only one performed on a 8m class telescope, was designed to target 100 000 stars using FLAMES on the ESO VLT (both Giraffe and UVES spectrographs), covering all the Milky Way populations, with a special focus on open star clusters. Aims. This article provides an overview of the survey implementation (observations, data quality, analysis and its success, data products, and releases), of the open cluster survey, of the science results and potential, and of the survey legacy. A companion article reviews the overall survey motivation, strategy, Giraffe pipeline data reduction, organisation, and workflow. Methods. We made use of the information recorded and archived in the observing blocks; during the observing runs; in a number of relevant documents; in the spectra and master catalogue of spectra; in the parameters delivered by the analysis nodes and the working groups; in the final catalogue; and in the science papers. Based on these sources, we critically analyse and discuss the output and products of the Survey, including science highlights. We also determined the average metallicities of the open clusters observed as science targets and of a sample of clusters whose spectra were retrieved from the ESO archive. Results. The Gaia-ESO Survey has determined homogeneous good-quality radial velocities and stellar parameters for a large fraction of its more than 110 000 unique target stars. Elemental abundances were derived for up to 31 elements for targets observed with UVES. Lithium abundances are delivered for about 1/3 of the sample. The analysis and homogenisation strategies have proven to be successful; several science topics have been addressed by the Gaia-ESO consortium and the community, with many highlight results achieved. Conclusions. The final catalogue will be released through the ESO archive in the first half of 2022, including the complete set of advanced data products. In addition to these results, the Gaia-ESO Survey will leave a very important legacy, for several aspects and for many years to come.

    Pengfei Li, Yong Tian, Mariana Júlio, Marcel Pawlowski, Federico Lelli, Stacy McGaugh, James Schombert, Justin Read, Po-Chieh Yu, Ko Chung-Ming (2023)Measuring galaxy cluster mass profiles into the low acceleration regime with galaxy kinematics, In: arXiv.org677A24 Cornell University Library, arXiv.org

    We probe the dynamical mass profiles of 10 galaxy clusters from the HIghest X-ray FLUx Galaxy Cluster Sample (HIFLUGCS) using galaxy kinematics. We numerically solve the spherical Jeans equation, and parameterize the dynamical mass profile and the galaxy velocity anisotropy profile using two general functions to ensure that our results are not biased towards any specific model. The mass-velocity anisotropy degeneracy is ameliorated by using two "virial shape parameters" that depend on the fourth moment of velocity distribution. The resulting velocity anisotropy estimates consistently show a nearly isotropic distribution in the inner regions, with an increasing radial anisotropy towards large radii. We compare our derived dynamical masses with those calculated from X-ray gas data assuming hydrostatic equilibrium, finding that massive and rich relaxed clusters generally present consistent mass measurements, while unrelaxed or low-richness clusters have systematically larger total mass than hydrostatic mass by an average of 50\%. This might help alleviate current tensions in the measurement of \(\sigma_8\), but it also leads to cluster baryon fractions below the cosmic value. Finally, our approach probes accelerations as low as \(10^{-11}\) m s\(^{-2}\), comparable to the outskirts of individual late-type galaxies. We confirm that galaxy clusters deviate from the radial acceleration relation defined by galaxies.

    Joel Berge, Laura Baudis, Philippe Brax, Sheng-Wey Chiow, Bruno Christophe, Olivier Dore, Pierre Fayet, Aurelien Hees, Philippe Jetzer, Claus Laemmerzahl, Meike List, Gilles Metris, Martin Pernot-Borras, Justin Read, Serge Reynaud, Jason Rhodes, Benny Rievers, Manuel Rodrigues, Timothy Sumner, Jean-Philippe Uzan, Nan Yu (2021)The local dark sector Probing gravitation's low-acceleration frontier and dark matter in the Solar System neighborhood, In: Experimental astronomy51(3)pp. 1737-1766 Springer Nature

    We speculate on the development and availability of new innovative propulsion techniques in the 2040s, that will allow us to fly a spacecraft outside the Solar System (at 150 AU and more) in a reasonable amount of time, in order to directly probe our (gravitational) Solar System neighborhood and answer pressing questions regarding the dark sector (dark energy and dark matter). We identify two closely related main science goals, as well as secondary objectives that could be fulfilled by a mission dedicated to probing the local dark sector: (i) begin the exploration of gravitation's low-acceleration regime with a spacecraft and (ii) improve our knowledge of the local dark matter and baryon densities. Those questions can be answered by directly measuring the gravitational potential with an atomic clock on-board a spacecraft on an outbound Solar System orbit, and by comparing the spacecraft's trajectory with that predicted by General Relativity through the combination of ranging data and the in-situ measurement (and correction) of non-gravitational accelerations with an on-board accelerometer. Despite a wealth of new experiments getting online in the near future, that will bring new knowledge about the dark sector, it is very unlikely that those science questions will be closed in the next two decades. More importantly, it is likely that it will be even more urgent than currently to answer them. Tracking a spacecraft carrying a clock and an accelerometer as it leaves the Solar System may well be the easiest and fastest way to directly probe our dark environment.

    Justin Read, P. Steger (2017)How to break the density-anisotropy degeneracy in spherical stellar systems, In: Monthly Notices of the Royal Astronomical Society471(4)pp. 4541-4558 Oxford University Press

    We present a new non-parametric Jeans code, GravSphere, that recovers the density ρ(r) and velocity anisotropy β(r) of spherical stellar systems, assuming only that they are in a steady-state. Using a large suite of mock data, we confirm that with only line-of-sight velocity data, GravSphere provides a good estimate of the density at the projected stellar half mass radius, ρ(R1/2), but is not able to measure ρ(r) or β(r), even with 10,000 tracer stars. We then test three popular methods for breaking this ρ − β degeneracy: using multiple populations with different R1/2; using higher order ‘Virial Shape Parameters’ (VSPs); and including proper motion data. We find that two populations provide an excellent recovery of ρ(r) in-between their respective R1/2. However, even with a total of ∼7, 000 tracers, we are not able to well-constrain β(r) for either population. By contrast, using 1000 tracers with higher order VSPs we are able to measure ρ(r) over the range 0.5 < r/R1/2 < 2 and broadly constrain β(r). Including proper motion data for all stars gives an even better performance, with ρ and β well-measured over the range 0.25 < r/R1/2 < 4. Finally, we test GravSphere on a triaxial mock galaxy that has axis ratios typical of a merger remnant, [1: 0.8: 0.6]. In this case, GravSphere can become slightly biased. However, we find that when this occurs the data are poorly fit, allowing us to detect when such departures from spherical symmetry become problematic.

    James Petts, Justin Read, Alessia Gualandris (2016)A semi-analytic dynamical friction model for cored galaxies, In: Monthly Notices of the Royal Astronomical Society463(1)pp. 858-869 Oxford University Press

    We present a dynamical friction model based on Chandrasekhar’s formula that reproduces the fast inspiral and stalling experienced by satellites orbiting galaxies with a large constant density core. We show that the fast inspiral phase does not owe to resonance. Rather, it owes to the background velocity distribution function for the constant density core being dissimilar from the usually-assumed Maxwellian distribution. Using the correct background velocity distribution function and the semi-analytic model from Petts, Gualandris & Read (2015), we are able to correctly reproduce the infall rate in both cored and cusped potentials. However, in the case of large cores, our model is no longer able to correctly capture core-stalling. We show that this stalling owes to the tidal radius of the satellite approaching the size of the core. By switching off dynamical friction when rt(r) = r (where rt is the tidal radius at the satellite’s position) we arrive at a model which reproduces the N-body results remarkably well. Since the tidal radius can be very large for constant density background distributions, our model recovers the result that stalling can occur for Ms/Menc 1, where Ms and Menc are the mass of the satellite and the enclosed galaxy mass, respectively. Finally, we include the contribution to dynamical friction that comes from stars moving faster than the satellite. This next-to-leading order effect becomes the dominant driver of inspiral near the core region, prior to stalling.

    Zhen Wan, William H Oliver, Geraint F Lewis, Justin Read, Michelle Collins (2019)On the origin of the asymmetric dwarf galaxy distribution around andromeda, In: Monthly Notices of the Royal Astronomical Society492(1)pp. 456-467 Oxford University Press

    The dwarf galaxy distribution surrounding M31 is significantly anisotropic in nature. Of the 30 dwarf galaxies in this distribution, 15 form a disc-like structure and 23 are contained within the hemisphere facing the Milky Way. Using a realistic local potential, we analyse the conditions required to produce and maintain these asymmetries. We find that some dwarf galaxies are required to have highly eccentric orbits in order to preserve the presence of the hemispherical asymmetry with an appropriately large radial dispersion. Under the assumption that the dwarf galaxies originate from a single association or accretion event, we find that the initial size and specific energy of that association must both be relatively large in order to produce the observed hemispherical asymmetry. However if the association was large in physical size, the very high-energy required would enable several dwarf galaxies to escape from the M31 and be captured by the Milky Way. Furthermore, we find that associations that result in this structure have total specific energies concentrated around E = V_esc2 - V_init2 ̃ 200^2 - 300^2 {km^2 s^{-2}}, implying that the initial velocity and initial position needed to produce the structure are strongly correlated. The overlap of initial conditions required to produce the radial dispersion, angular dispersion, and the planar structure is small and suggests that either they did not originate from a single accretion event, or that these asymmetric structures are short-lived.

    Martin P. Rey, Andrew Pontzen, Oscar Agertz, Matthew Orkney, Justin Read, Amélie Saintonge, Christian Pedersen (2019)EDGE: The Origin of Scatter in Ultra-faint Dwarf Stellar Masses and Surface Brightnesses, In: Astrophysical Journal Letters886(1)L3 The American Astronomical Society

    We demonstrate how the least luminous galaxies in the universe, ultra-faint dwarf galaxies, are sensitive to their dynamical mass at the time of cosmic reionization. We select a low-mass (~ ´ 1.5 10 M☉ 9 ) dark matter halo from a cosmological volume, and perform zoom hydrodynamical simulations with multiple alternative histories using “genetically modified” initial conditions. Earlier-forming ultra-faints have higher stellar mass today, due to a longer period of star formation before their quenching by reionization. Our histories all converge to the same final dynamical mass, demonstrating the existence of extended scatter (1 dex) in stellar masses at fixed halo mass due to the diversity of possible histories. One of our variants builds less than 2% of its final dynamical mass before reionization, rapidly quenching in situ star formation. The bulk of its final stellar mass is later grown by dry mergers, depositing stars in the galaxy’s outskirts and hence expanding its effective radius. This mechanism constitutes a new formation scenario for highly diffuse (r1 2 ~ 820 pc, ~ - 32 mag arcsec 2 ), metal-poor ([Fe H 2.9 ] = - ), ultra-faint (V = -5.7) dwarf galaxies within the reach of next-generation low surface brightness surveys.

    A Rojas-Nino, Justin Read, A Aguilar, Maxime Delorme (2016)An efficient positive potential-density pair expansion for modelling galaxies, In: Monthly Notices of the Royal Astronomical Society459(3)pp. 3349-3355 Oxford University Press

    We present a novel positive potential-density pair expansion for modelling galaxies, based on the Miyamoto–Nagai disc. By using three sets of such discs, each one of them aligned along each symmetry axis, we are able to reconstruct a broad range of potentials that correspond to density profiles from exponential discs to 3D power-law models with varying triaxiality (henceforth simply ‘twisted’ models). We increase the efficiency of our expansion by allowing the scalelength parameter of each disc to be negative. We show that, for suitable priors on the scalelength and scaleheight parameters, these ‘MNn discs’ (Miyamoto–Nagai negative) have just one negative density minimum. This allows us to ensure global positivity by demanding that the total density at the global minimum is positive. We find that at better than 10 per cent accuracy in our density reconstruction, we can represent a radial and vertical exponential disc over 0.1–10 scalelengths/scaleheights with four MNn discs; a Navarro, Frenk and White (NFW) profile over 0.1–10 scalelengths with four MNn discs; and a twisted triaxial NFW profile with three MNn discs per symmetry axis. Our expansion is efficient, fully analytic, and well suited to reproducing the density distribution and gravitational potential of galaxies from discs to ellipsoids.

    Oscar Agertz, Andrew Pontzen, Justin Read, Martin P Rey, Matthew Orkney, Joakim Rosdahl, Romain Teyssier, Robbert Verbeke, Michael Kretschmer, Sarah Nickerson (2020)EDGE: the mass–metallicity relation as a critical test of galaxy formation physics, In: Monthly Notices of the Royal Astronomical Society491(2)pp. 1656-1672 Oxford University Press

    We introduce the 'Engineering Dwarfs at Galaxy Formation's Edge' (EDGE) project to study the cosmological formation and evolution of the smallest galaxies in the Universe. In this first paper, we explore the effects of resolution and sub-grid physics on a single low-mass halo (M_halo=109{ M}_☉), simulated to redshift z = 0 at a mass and spatial resolution of ̃ 20{ M}_☉ and ∼3 pc. We consider different star formation prescriptions, supernova feedback strengths, and on-the-fly radiative transfer (RT). We show that RT changes the mode of galactic self-regulation at this halo mass, suppressing star formation by causing the interstellar and circumgalactic gas to remain predominantly warm (∼104 K) even before cosmic reionization. By contrast, without RT, star formation regulation occurs only through starbursts and their associated vigorous galactic outflows. In spite of this difference, the entire simulation suite (with the exception of models without any feedback) matches observed dwarf galaxy sizes, velocity dispersions, V-band magnitudes, and dynamical mass-to-light-ratios. This is because such structural scaling relations are predominantly set by the host dark matter halo, with the remaining model-to-model variation being smaller than the observational scatter. We find that only the stellar mass-metallicity relation differentiates the galaxy formation models. Explosive feedback ejects more metals from the dwarf, leading to a lower metallicity at a fixed stellar mass. We conclude that the stellar mass-metallicity relation of the very smallest galaxies provides a unique constraint on galaxy formation physics.

    Ricardo Carrera, Blair C. Conn, Noelia Noel, Justin Read, Ángel R López Sánchez (2017)The Magellanic Inter-Cloud Project (MAGIC) III: First spectroscopic evidence of a dwarf stripping a dwarf, In: Monthly Notices of the Royal Astronomical Society471(4)pp. 4571-4578 Oxford University Press

    The Magellanic Bridge (MB) is a gaseous stream that links the Large (LMC) and Small (SMC) Magellanic Clouds. Current simulations suggest that the MB forms from a recent interaction between the Clouds. In this scenario, the MB should also have an associated stellar bridge formed by stars tidally stripped from the SMC by the LMC. There are several observational evidences for these stripped stars, from the presence of intermediate age populations in the MB and carbon stars, to the recent observation of an over-density of RR Lyrae stars offset from the MB. However, spectroscopic confirmation of stripped stars in the MB remains lacking. In this paper, we use medium resolution spectra to derive the radial velocities and metallicities of stars in two fields along the MB. We show from both their chemistry and kinematics that the bulk of these stars must have been tidally stripped from the SMC. This is the first spectroscopic evidence for a dwarf galaxy being tidally stripped by a larger dwarf.

    J I Read, M G Walker, P Steger (2019)Dark matter heats up in dwarf galaxies, In: Monthly Notices of the Royal Astronomical Society484(1)pp. 1401-1420

    Gravitational potential fluctuations driven by bursty star formation can kinematically ‘heat up’ dark matter at the centres of dwarf galaxies. A key prediction of such models is that, at a fixed dark matter halo mass, dwarfs with a higher stellar mass will have a lower central dark matter density. We use stellar kinematics and HI gas rotation curves to infer the inner dark matter densities of eight dwarf spheroidal and eight dwarf irregular galaxies with a wide range of star formation histories. For all galaxies, we estimate the dark matter density at a common radius of 150 pc, ρDM(150pc)⁠. We find that our sample of dwarfs falls into two distinct classes. Those that stopped forming stars over 6 Gyr ago favour central densities ρDM(150pc)>108 M⊙ kpc−3, consistent with cold dark matter cusps, while those with more extended star formation favour ρDM(150pc)

    Sukanya Chakrabarti, Philip Chang, Adrian M. Price-Whelan, Justin Read, Leo Blitz, Lars Hernquist (2019)Antlia 2’s Role in Driving the Ripples in the Outer Gas Disk of the Galaxy, In: The Astrophysical Journal886(1)pp. 67-73

    We employ the earlier published proper motions of the newly discovered Antlia 2 dwarf galaxy derived from Gaia data to calculate its orbital distribution in the cosmologically recent past. Using these observationally motivated orbits, we calculate the effect of the Antlia 2 dwarf galaxy on the outer H I disk of the Milky Way, using both test particle and smoothed particle hydrodynamics simulations. We find that orbits with low pericenters, ∼10 kpc, produce disturbances that match the observed outer H I disk perturbations. We have independently recalculated the proper motion of the Antlia 2 dwarf from Gaia data and found a proper motion of (μ α cosδ, μ δ ) = (−0.068, 0.032) ± (0.023, −0.031) mas yr−1, which agrees with results from Torrealba et al. within the errors, but gives lower mean pericenters, e.g., ∼15 kpc for our fiducial model of the Milky Way. We also show that the Sagittarius dwarf galaxy interaction does not match the observed perturbations in the outer gas disk. Thus, Antlia 2 may be the driver of the observed large perturbations in the outer gas disk of the Galaxy. The current location of the Antlia 2 dwarf galaxy closely matches that predicted by an earlier dynamical analysis of the dwarf galaxy that drove ripples in the outer Galaxy, and, in particular, its orbit is nearly coplanar to the Galactic disk. If the Antlia 2 dwarf galaxy is responsible for the perturbations in the outer Galactic disk, it would have a specific range of proper motions that we predict here; this can be tested soon with Gaia DR-3 and Gaia DR-4 data.

    J. I. Read, D. Erkal (2019)Abundance matching with the mean star formation rate: there is no missing satellites problem in the Milky Way above M200∼109M, In: Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP)

    We introduce a novel abundance matching technique that produces a more accurate estimate of the pre-infall halo mass, M200, for satellite galaxies. To achieve this, we abundance match with the mean star formation rate, averaged over the time when a galaxy was forming stars, ⟨SFR⟩, instead of the stellar mass, M∗. Using data from the Sloan Digital Sky Survey, the GAMA survey and the Bolshoi simulation, we obtain a statistical ⟨SFR⟩−M200 relation in ΛCDM. We then compare the pre-infall halo mass, Mabund200, derived from this relation with the pre-infall dynamical mass, Mdyn200, for 21 nearby dSph and dIrr galaxies, finding a good agreement between the two. As a first application, we use our new ⟨SFR⟩−M200 relation to empirically measure the cumulative mass function of a volume-complete sample of bright Milky Way satellites within 280 kpc of the Galactic centre. Comparing this with a suite of cosmological 'zoom' simulations of Milky Way-mass halos that account for subhalo depletion by the Milky Way disc, we find no missing satellites problem above M200∼09M⊙ in the Milky Way. We discuss how this empirical method can be applied to a larger sample of nearby spiral galaxies.

    Justin Read, G Iorio, Oscar Agertz, F Fraternali (2017)The stellar mass–halo mass relation of isolated field dwarfs: a critical test of ΛCDM at the edge of galaxy formation, In: Monthly Notices of the Royal Astronomical Society467(2)pp. 2019-2038 Oxford University Press

    We fit the rotation curves of isolated dwarf galaxies to directly measure the stellar mass-halo mass relation (M∗ − M200) over the mass range 5 × 105

    S Sivertsson, H Silverwood, J I Read, G Bertone, P Steger (2018)The localdark matter density from SDSS-SEGUE G-dwarfs, In: Monthly Notices of the Royal Astronomical Society478(2)pp. 1677-1693 Oxford University Press (OUP)

    We derive the local dark matter density by applying the integrated Jeans equation method from Silverwood et al. to SDSS-SEGUE G-dwarf data processed and presented by Büdenbender et al. We use the MULTINEST Bayesian nested sampling software to fit a model for the baryon distribution, dark matter, and tracer stars, including a model for the `tilt term' that couples the vertical and radial motions, to the data. The α-young population from Büdenbender et al. yields the most reliable result of ρ_dm= 0.46^{+0.07}_{-0.08} {GeV cm}^{-3}= 0.012^{+0.002}_{-0.002} M_{☉} pc^{-3}. Our analyses yield inconsistent results for the α-young and α-old data, pointing to problems in the tilt term and its modelling, the data itself, the assumption of a flat rotation curve, or the effects of disequilibria.

    J I Read, M G Walker, P Steger (2018)The case for a cold dark matter cusp in Draco, In: Monthly Notices of the Royal Astronomical Society481(1)pp. 860-877 Oxford University Press (OUP)

    We use a new mass modelling method, GRAVSPHERE, to measure the central dark matter density profile of the Draco dwarf spheroidal galaxy. Draco's star formation shut down long ago, making it a prime candidate for hosting a `pristine' dark matter cusp, unaffected by stellar feedback during galaxy formation. We first test GRAVSPHERE on a suite of tidally stripped mock `Draco'-like dwarfs. We show that we are able to correctly infer the dark matter density profile of both cusped and cored mocks within our 95 per cent confidence intervals. While we obtain only a weak inference on the logarithmic slope of these density profiles, we are able to obtain a robust inference of the amplitude of the inner dark matter density at 150 pc, ρ _DM(150 pc). We show that, combined with constraints on the density profile at larger radii, this is sufficient to distinguish a Λ Cold Dark Matter (ΛCDM) cusp - that has ρ _DM(150 pc) ≳ 1.8 × 10^8 M_☉ kpc^{-3} - from alternative dark matter models that have lower inner densities. We then apply GRAVSPHERE to the real Draco data. We find that Draco has an inner dark matter density of ρ _DM(150 pc) = 2.4_{-0.6}^{+0.5} × 10^8 M_☉ kpc^{-3}, consistent with a ΛCDM cusp. Using a velocity-independent SIDM model, calibrated on ΛSIDM cosmological simulations, we show that Draco's high central density gives an upper bound on the SIDM cross-section of σ/m < 0.57 cm2 g-1 at 99 per cent confidence. We conclude that the inner density of nearby dwarf galaxies like Draco provides a new and competitive probe of dark matter models.

    Kearn Grisdale, Oscar Agertz, AB Romeo, Florent Renaud, Justin Read (2016)The impact of stellar feedback on the density and velocity structure of the interstellar medium, In: Monthly Notices of the Royal Astronomical Society466(1)pp. 1093-1110 Oxford University Press

    We study the impact of stellar feedback in shaping the density and velocity structure of neutral hydrogen (H I) in disc galaxies. For our analysis, we carry out ∼4.6 pc resolution N-body+adaptive mesh refinement hydrodynamic simulations of isolated galaxies, set up to mimic a Milky Way and a Large and Small Magellanic Cloud. We quantify the density and velocity structure of the interstellar medium using power spectra and compare the simulated galaxies to observed H I in local spiral galaxies from THINGS (The H I Nearby Galaxy Survey). Our models with stellar feedback give an excellent match to the observed THINGS H I density power spectra. We find that kinetic energy power spectra in feedback-regulated galaxies, regardless of galaxy mass and size, show scalings in excellent agreement with supersonic turbulence (E(k) ∝ k-2) on scales below the thickness of the H I layer. We show that feedback influences the gas density field, and drives gas turbulence, up to large (kpc) scales. This is in stark contrast to density fields generated by large-scale gravity-only driven turbulence. We conclude that the neutral gas content of galaxies carries signatures of stellar feedback on all scales.

    Cecilia Mateu, Justin I Read, Daisuke Kawata (2017)Fourteen candidate RR Lyrae star streams in the inner Galaxy, In: Monthly Notices of the Royal Astronomical Society474(3)pp. 4112-4129 Oxford University Press (OUP)

    We apply the GC3 stream-finding method to RR Lyrae stars (RRLSs) in the Catalina survey. We find 2 RRLS stream candidates at >4σ confidence and another 12 at >3.5σ confidence over the Galactocentric distance range 4 < D/kpc < 26. Of these, only two are associated with known globular clusters (NGC 1261 and Arp2). The remainder are candidate `orphan' streams, consistent with the idea that globular cluster streams are most visible close to dissolution. Our detections are likely a lower bound on the total number of dissolving globulars in the inner galaxy, since many globulars have few RRLSs, while only the brightest streams are visible over the Galactic RRLS background, particularly given the current lack of kinematical information. We make all of our candidate streams publicly available and provide a new galstreamsPYTHON library for the footprints of all known streams and overdensities in the Milky Way.

    José R Bermejo-Climent, Giuseppina Battaglia, Carme Gallart, Arianna Di Cintio, Chris B Brook, Luis Cicuéndez, Matteo Monelli, Ryan Leaman, Lucio Mayer, Jorge Peñarrubia, Justin I Read (2018)On the early evolution of Local Group dwarf galaxy types: star formation and supernova feedback, In: Monthly Notices of the Royal Astronomical Society479(2)pp. 1514-1527 Oxford University Press (OUP)

    According to star formation histories (SFHs), Local Group dwarf galaxies can be broadly classified in two types: those forming most of their stars before z = 2 (fast) and those with more extended SFHs (slow). The most precise SFHs are usually derived from deep but not very spatially extended photometric data; this might alter the ratio of old to young stars when age gradients are present. Here, we correct for this effect and derive the mass formed in stars by z = 2 for a sample of 16 Local Group dwarf galaxies. We explore early differences between fast and slow dwarfs, and evaluate the impact of internal feedback by supernovae (SNe) on the baryonic and dark matter (DM) component of the dwarfs. Fast dwarfs assembled more stellar mass at early times and have larger amounts of DM within the half-light radius than slow dwarfs. By imposing that slow dwarfs cannot have lost their gas by z = 2, we constrain the maximum coupling efficiency of SN feedback to the gas and to the DM to be ̃10 per cent. We find that internal feedback alone appears insufficient to quench the SFH of fast dwarfs by gas deprivation, in particular for the fainter systems. Nonetheless, SN feedback can core the DM halo density profiles relatively easily, producing cores of the sizes of the half-light radius in fast dwarfs by z = 2 with very low efficiencies. Amongst the `classical' Milky Way satellites, we predict that the smallest cores should be found in Draco and Ursa Minor, while Sculptor and Fornax should host the largest ones.

    Justin Read, G Iorio, Oscar Agertz, F Fraternali (2016)Understanding the shape and diversity of dwarf galaxy rotation curves in LCDM, In: Monthly Notices of the Royal Astronomical Society462(4)pp. 3628-3645 Oxford University Press

    The shape and wide diversity of dwarf galaxy rotation curves is at apparent odds with dark matter halos in LCDM. We generate mock rotation curve data from dwarf galaxy simulations to show that this owes to bursty star formation driven by stellar feedback. There are three main effects. Firstly, stellar feedback transforms dark matter cusps into cores. Ignoring such transformations leads to a poor fit of the rotation curve shape and a large systematic bias on the halo concentration parameter c. Secondly, if close to a recent starburst, large HI bubbles push the rotation curve out of equilibrium. This makes the gas rotational velocity a poor probe of the underlying potential, leading to a systematic error on the halo virial mass M200 of up to half a dex. Thirdly, when galaxies are viewed near face-on (i

    JA Petts, A Gualandris, JI Read (2015)A semi-analytic dynamical friction model that reproduces core stalling, In: MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY454(4)pp. 3778-3791 OXFORD UNIV PRESS
    A Gualandris, JI Read, W Dehnen, E Bortolas (2017)Collisionless loss-cone refilling: there is no final parsec problem, In: Monthly Notices of the Royal Astronomical Society464(2)pp. 2301-2310

    Coalescing massive black hole binaries, formed during galaxy mergers, are expected to be a primary source of low-frequency gravitational waves. Yet in isolated gas-free spherical stellar systems, the hardening of the binary stalls at parsec-scale separations owing to the inefficiency of relaxation-driven loss-cone refilling. Repopulation via collisionless orbit diffusion in triaxial systems is more efficient, but published simulation results are contradictory. While sustained hardening has been reported in simulations of galaxy mergers with N ∼ 106 stars and in early simulations of rotating models, in isolated non-rotating triaxial models the hardening rate continues to fall with increasing N, a signature of spurious two-body relaxation. We present a novel approach for studying loss-cone repopulation in galactic nuclei. Since loss-cone repopulation in triaxial systems owes to orbit diffusion, it is a purely collisionless phenomenon and can be studied with an approximated force calculation technique, provided the force errors are well behaved and sufficiently small. We achieve this using an accurate fast multipole method and define a proxy for the hardening rate that depends only on stellar angular momenta. We find that the loss cone is efficiently replenished even in very mildly triaxial models (with axis ratios 1:0.9:0.8). Such triaxiality is unavoidable following galactic mergers and can drive binaries into the gravitational wave regime. We conclude that there is no ‘final parsec problem’.

    Imran Nasim, Alessia Gualandris, Justin Read, Walter Dehnen, Maxime Delorme, Fabio Antonini (2020)Defeating stochasticity: coalescence timescales of massive black holes in galaxy mergers, In: Monthly Notices of the Royal Astronomical Society Oxford University Press

    The coalescence of massive black hole binaries (BHBs) in galactic mergers is the primary source of gravitational waves (GWs) at low frequencies. Current estimates of GW detection rates for the Laser Interferometer Space Antenna and the Pulsar Timing Array vary by three orders of magnitude. To understand this variation, we simulate the merger of equal-mass, eccentric, galaxy pairs with central massive black holes and shallow inner density cusps. We model the formation and hardening of a central BHB using the Fast Multiple Method as a force solver, which features a O¹Nº scaling with the number N of particles and obtains results equivalent to direct-summation simulations. At N 5105, typical for contemporary studies, the eccentricity of the BHBs can vary significantly for different random realisations of the same initial condition, resulting in a substantial variation of the merger timescale. This scatter owes to the stochasticity of stellar encounters with the BHB and decreases with increasing N. We estimate that N 107 within the stellar half-light radius suffices to reduce the scatter in the merger timescale to 10%. Our results suggest that at least some of the uncertainty in low-frequency GW rates owes to insufficient numerical resolution.

    Elisa Bortolas, Alessia Gualandris, Massimo Dotti, Justin I. Read (2018)The influence of Massive Black Hole Binaries on the Morphology of Merger Remnants, In: Monthly Notices of the Royal Astronomical Society477(2)pp. 2310-2325 Oxford University Press (OUP)

    Massive black hole (MBH) binaries, formed as a result of galaxy mergers, are expected to harden by dynamical friction and three-body stellar scatterings, until emission of gravitational waves (GWs) leads to their final coalescence. According to recent simulations, MBH binaries can efficiently harden via stellar encounters only when the host geometry is triaxial, even if only modestly, as angular momentum diffusion allows an efficient repopulation of the binary loss cone. In this paper, we carry out a suite of N-body simulations of equal-mass galaxy collisions, varying the initial orbits and density profiles for the merging galaxies and running simulations both with and without central MBHs. We find that the presence of an MBH binary in the remnant makes the system nearly oblate, aligned with the galaxy merger plane, within a radius enclosing 100 MBH masses. We never find binary hosts to be prolate on any scale. The decaying MBHs slightly enhance the tangential anisotropy in the centre of the remnant due to angular momentum injection and the slingshot ejection of stars on nearly radial orbits. This latter effect results in about 1% of the remnant stars being expelled from the galactic nucleus. Finally, we do not find any strong connection between the remnant morphology and the binary hardening rate, which depends only on the inner density slope of the remnant galaxy. Our results suggest that MBH binaries are able to coalesce within a few Gyr, even if the binary is found to partially erase the merger-induced triaxiality from the remnant.

    JI Read, MI Wilkinson, NW Evans, G Gilmore, JT Kleyna (2006)The importance of tides for the Local Group dwarf spheroidals, In: _mnras367pp. 387-399
    NED Noel, BC Conn, JI Read, R Carrera, A Dolphin, H-W Rix (2015)The MAGellanic Inter-Cloud (MAGIC) project - II. Slicing up the Bridge, In: Monthly Notices of the Royal Astronomical Society452(4)pp. 4222-4235 Oxford University Press

    The origin of the gas in between the Magellanic Clouds (MCs), known as the Magellanic Bridge, has always been the subject of controversy. To shed light into this, we present the results from the MAGellanic Inter-Cloud II (MAGIC II) project aimed at probing the stellar populations in 10 large fields located perpendicular to the main ridge-line of H i in the Inter-Cloud region. We secured these observations of the stellar populations in between the MCs using the WFI (Wide Field Imager) camera on the 2.2 m telescope in La Silla. Using colour–magnitude diagrams, we trace stellar populations across the Inter-Cloud region. In good agreement with MAGIC I, we find significant intermediate-age stars in the Inter-Cloud region as well as young stars of a similar age to the last pericentre passage in between the MCs (∼200 Myr ago). We show here that the young, intermediate-age and old stars have distinct spatial distributions. The young stars correlate well with the H i gas suggesting that they were either recently stripped from the Small Magellanic Cloud (SMC) or formed in situ. The bulk of intermediate-age stars are located mainly in the Bridge region where the H i column density is higher, but they are more spread out than the young stars. They have very similar properties to stars located ∼2 kpc from the SMC centre, suggesting that they were tidally stripped from this region. Finally, the old stars extend to some 8 kpc from the SMC supporting the idea that all galaxies have a large extended metal-poor stellar halo.

    JI Read, T Goerdt, B Moore, AP Pontzen, J Stadel, G Lake (2006)Dynamical friction in constant density cores: a failure of the Chandrasekhar formula, In: _mnras373pp. 1451-1460
    H Silverwood, S Sivertsson, P Steger, Justin Read, G Bertone (2016)A non-parametric method for measuring the local dark matter density, In: Monthly Notices of the Royal Astronomical Society Main Journal.459(4)pp. 4191-4208 Oxford University Press

    We present a new method for determining the local dark matter density using kinematic data for a population of tracer stars. The Jeans equation in the z-direction is integrated to yield an equation that gives the velocity dispersion as a function of the total mass density, tracer density, and the ‘tilt’ term that describes the coupling of vertical and radial motions. We then fit a dark matter mass profile to tracer density and velocity dispersion data to derive credible regions on the vertical dark matter density profile. Our method avoids numerical differentiation, leading to lower numerical noise, and is able to deal with the tilt term while remaining one dimensional. In this study we present the method and perform initial tests on idealised mock data. We also demonstrate the importance of dealing with the tilt term for tracers that sample > ∼ 1 kpc above the disc plane. If ignored, this results in a systematic underestimation of the dark matter density.

    Ramon Rey-Raposo, Justin I Read (2018)The alignment is in their stars: on the spin-alignment of stars in star clusters, In: Monthly Notices of the Royal Astronomical Society: Letters481(1)pp. L16-L20 Oxford University Press (OUP)

    We simulate star formation in two molecular clouds extracted from a larger disc-galaxy simulation with a spatial resolution of ∼0.1 pc, one exiting a spiral arm dominated by compression, and another in an inter-arm region more strongly affected by galactic shear. Treating the stars as ‘sink particles’, we track their birth angular momentum, and the later evolution of their angular momentum due to gas accretion. We find that in both clouds, the sinks have spin vectors that are aligned with one another, and with the global angular momentum vector of the star cluster. This alignment is present at birth, but enhanced by later gas accretion. In the compressive cloud, the sink-spins remain aligned with the gas for at least a free-fall time. By contrast, in the shear cloud, the increased turbulent mixing causes the sinks to rapidly misalign with their birth cloud on approximately a gas free-fall time. In spite of this, both clouds show a strong alignment of sink-spins at the end of our simulations, independently of environment.

    Michele De Leo, Ricardo Carrera, Noelia E.D Noel, Justin I. Read, Denis Erkal, Carme Gallart (2020)Revealing the tidal scars of the Small Magellanic Cloud, In: Monthly Notices of the Royal Astronomical Society495(1)pp. 98-113 Oxford University Press (OUP)

    Due to their close proximity, the Large and Small Magellanic Clouds (LMC/SMC) provide natural laboratories for understanding how galaxies form and evolve. With the goal of determining the structure and dynamical state of the SMC, we present new spectroscopic data for ∼3000 SMC red giant branch stars observed using the AAOmega spectrograph at the Anglo-Australian Telescope. We complement our data with further spectroscopic measurements from previous studies that used the same instrumental configuration as well as proper motions from the Gaia Data Release 2 catalogue. Analysing the photometric and stellar kinematic data, we find that the SMC centre of mass presents a conspicuous offset from the velocity centre of its associated H i gas, suggesting that the SMC gas is likely to be far from dynamical equilibrium. Furthermore, we find evidence that the SMC is currently undergoing tidal disruption by the LMC within 2 kpc of the centre of the SMC, and possibly all the way into the very core. This is revealed by a net outward motion of stars from the SMC centre along the direction towards the LMC and an apparent tangential anisotropy at all radii. The latter is expected if the SMC is undergoing significant tidal stripping, as we demonstrate using a suite of N-body simulations of the SMC/LMC system disrupting around the Milky Way. Our results suggest that dynamical models for the SMC that assume a steady state will need to be revisited.

    P Saha, JI Read, LLR Williams (2006)Two Strong-Lensing Clusters Confront Universal Dark Matter Profiles, In: _apjl652pp. L5-L8
    T Goerdt, B Moore, JI Read, J Stadel, M Zemp (2006)Does the Fornax dwarf spheroidal have a central cusp or core?, In: _mnras368pp. 1073-1077
    JI Read, P Saha, AV Macciò (2007)Radial Density Profiles of Time-Delay Lensing Galaxies, In: _apj667pp. 645-654
    JI Read, AP Pontzen, M Viel (2006)On the formation of dwarf galaxies and stellar haloes, In: _mnras371pp. 885-897
    JI Read, MI Wilkinson, NW Evans, G Gilmore, JT Kleyna (2006)The tidal stripping of satellites, In: _mnras366pp. 429-437
    W Dehnen, JI Read (2011)N-body simulations of gravitational dynamics, In: European Physical Journal Plus126pp. 55-55
    M D A Orkney, J I Read, J A Petts, M Gieles (2019)Globular clusters as probes of dark matter cusp-core transformations, In: Monthly Notices of the Royal Astronomical Society488(3)pp. 2977-2988 Oxford University Press (OUP)

    Bursty star formation in dwarf galaxies can slowly transform a steep dark matter cusp into a constant density core. We explore the possibility that globular clusters (GCs) retain a dynamical memory of this transformation. To test this, we use the NBODY6DF code to simulate the dynamical evolution of GCs, including stellar evolution, orbiting in static and time-varying potentials for a Hubble time. We find that GCs orbiting within a cored dark matter halo, or within a halo that has undergone a cusp-core transformation, grow to a size that is substantially larger (Reff ˃ 10 pc) than those in a static cusped dark matter halo. They also produce much less tidal debris. We find that the cleanest signal of an historic cusp-core transformation is the presence of large GCs with tidal debris. However, the effect is small and will be challenging to observe in real galaxies. Finally, we qualitatively compare our simulated GCs with the observed GC populations in the Fornax, NGC 6822, IKN, and Sagittarius dwarf galaxies. We find that the GCs in these dwarf galaxies are systematically larger (⟨Reff⟩ ≃ 7.8 pc), and have substantially more scatter in their sizes than in situ metal-rich GCs in the Milky Way and young massive star clusters forming in M83 (⟨Reff⟩ ≃ 2.5 pc). We show that the size, scatter, and survival of GCs in dwarf galaxies are all consistent with them having evolved in a constant density core, or a potential that has undergone a cusp-core transformation, but not in a dark matter cusp.

    S Garbari, JI Read, G Lake (2011)Limits on the local dark matter density, In: _mnras416pp. 2318-2340
    G. Iorio, F. Fraternali, C. Nipoti, E. Di Teodoro, Justin Read, G. Battaglia (2016)LITTLE THINGS in 3D: robust determination of the circular velocity of dwarf irregular galaxies., In: Monthly Notices of the Royal Astronomical Society466(4)pp. 4159-4192

    Dwarf irregular galaxies (dIrrs) are the smallest stellar systems with extended H I discs. The study of the kinematics of such discs is a powerful tool to estimate the total matter distribution at these very small scales. In this work, we study the H I kinematics of 17 galaxies extracted from the ‘Local Irregulars That Trace Luminosity Extremes, The H I Nearby Galaxy Survey’ (LITTLE THINGS). Our approach differs significantly from previous studies in that we directly fit 3D models (two spatial dimensions plus one spectral dimension) using the software 3DBAROLO, fully exploiting the information in the H I data cubes. For each galaxy, we derive the geometric parameters of the H I disc (inclination and position angle), the radial distribution of the surface density, the velocity-dispersion (σv) profile and the rotation curve. The circular velocity (Vc), which traces directly the galactic potential, is then obtained by correcting the rotation curve for the asymmetric drift. As an initial application, we show that these dIrrs lie on a baryonic Tully–Fisher relation in excellent agreement with that seen on larger scales. The final products of this work are high-quality, ready-to-use kinematic data (Vc and σv) that we make publicly available. These can be used to perform dynamical studies and improve our understanding of these low-mass galaxies.

    T Goerdt, B Moore, JI Read, J Stadel (2010)Core Creation in Galaxies and Halos Via Sinking Massive Objects, In: _apj725pp. 1707-1716
    NR Tanvir, AD Mackey, AMN Ferguson, A Huxor, JI Read, GF Lewis, MJ Irwin, S Chapman, R Ibata, MI Wilkinson, AW McConnachie, NF Martin, MB Davies, TJ Bridges (2012)The structure of star clusters in the outer halo of M31, In: _mnras422pp. 162-184
    R Teyssier, A Pontzen, Y Dubois, JI Read (2013)Cusp-core transformations in dwarf galaxies: observational predictions, In: _mnras429pp. 3068-3078
    J-H Kim, T Abel, O Agertz, GL Bryan, D Ceverino, C Christensen, C Conroy, A Dekel, NY Gnedin, NJ Goldbaum, J Guedes, O Hahn, A Hobbs, PF Hopkins, CB Hummels, F Iannuzzi, D Keres, A Klypin, AV Kravtsov, MR Krumholz, M Kuhlen, SN Leitner, P Madau, L Mayer, CE Moody, K Nagamine, ML Norman, J Onorbe, BW O'Shea, A Pillepich, JR Primack, T Quinn, JI Read, BE Robertson, M Rocha, DH Rudd, S Shen, BD Smith, AS Szalay, R Teyssier, R Thompson, K Todoroki, MJ Turk, JW Wadsley, JH Wise, A Zolotov (2014)THE AGORA HIGH-RESOLUTION GALAXY SIMULATIONS COMPARISON PROJECT, In: ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES210(1)ARTN 14 IOP PUBLISHING LTD
    A Knebe, SR Knollmann, SI Muldrew, FR Pearce, MA Aragon-Calvo, Y Ascasibar, PS Behroozi, D Ceverino, S Colombi, J Diemand, K Dolag, BL Falck, P Fasel, J Gardner, S Gottlöber, C-H Hsu, F Iannuzzi, A Klypin, Z Lukić, M Maciejewski, C McBride, MC Neyrinck, S Planelles, D Potter, V Quilis, Y Rasera, JI Read, PM Ricker, F Roy, V Springel, J Stadel, G Stinson, PM Sutter, V Turchaninov, D Tweed, G Yepes, M Zemp (2011)Haloes gone MAD: The Halo-Finder Comparison Project, In: _mnras415pp. 2293-2318
    Alexandra L Gregory, Michelle L M Collins, Justin Read, Michael J Irwin, Rodrigo A Ibata, Nicolas F Martin, Alan W McConnachie, Daniel R Weisz (2019)Kinematics of the Tucana Dwarf Galaxy: an unusually dense dwarf in the Local Group, In: Monthly Notices of the Royal Astronomical Society485(2)pp. 2010-2025 Oxford University Press (OUP)

    We present new FLAMES+GIRAFFE spectroscopy of 36 member stars in the isolated Local Group dwarf spheroidal galaxy Tucana. We measure a systemic velocity for the system of vTuc=216.7+2.9−2.8vTuc=216.7−2.8+2.9 km s−1, and a velocity dispersion of σv,Tuc=14.4+2.8−2.3σv,Tuc=14.4−2.3+2.8 km s−1. We also detect a rotation gradient of dvrdχ=7.6+4.2−4.3dvrdχ=7.6−4.3+4.2 km s−1 kpc−1, which reduces the systemic velocity to vTuc=215.2+2.8−2.7vTuc=215.2−2.7+2.8 km s−1 and the velocity dispersion to σv,Tuc=13.3+2.7−2.3σv,Tuc=13.3−2.3+2.7 km s−1. We perform Jeans modelling of the density profile of Tucana, using the line-of-sight velocities of the member stars. We find that it favours a high central density consistent with ‘pristine’ subhaloes in Λ cold dark matter, and a massive dark matter halo (∼1010 M⊙) consistent with expectations from abundance matching. Tucana appears to be significantly more centrally dense than other isolated Local Group dwarfs, making it an ideal laboratory for testing dark matter models.

    Ioana Ciucă, Daisuke Kawata, Shin’ichiro Ando, Francesca Calore, Justin I Read, Cecilia Mateu (2018)A Gaia DR2 search for dwarf galaxies towards Fermi-LAT sources: implications for annihilating dark matter, In: Monthly Notices of the Royal Astronomical Society480(2)pp. 2284-2291 Oxford University Press (OUP)

    We make the first attempt to find dwarf galaxies in eight Fermi-LAT extended, unassociated, source fields using Gaia DR2. After probing previously unexplored heliocentric distances of d ˂ 20 kpc with an extreme-deconvolution (XD) technique, we find no sign of a dwarf galaxy in any of these fields despite Gaia's excellent astrometric accuracy. Our detection limits are estimated by applying the XD method to mock data, obtaining a conservative limit on the stellar mass of M* ˂ 104 M☉ for d ˂ 20 kpc. Such a low stellar mass implies either a low-mass subhalo or a massive stripped-down subhalo. We use an analytic model for stripped subhaloes to argue that, given the sizes and fluxes of the Fermi-LAT sources, we can reject the hypothesis that they owe to dark matter annihilation.

    A Charbonnier, C Combet, M Daniel, S Funk, JA Hinton, D Maurin, C Power, JI Read, S Sarkar, MG Walker, MI Wilkinson (2011)Dark matter profiles and annihilation in dwarf spheroidal galaxies: prospectives for present and future γ-ray observatories - I. The classical dwarf spheroidal galaxies, In: _mnras418pp. 1526-1556
    H Lux, JI Read, G Lake (2010)Determining orbits for the Milky Way’s dwarfs, In: _mnras406pp. 2312-2324
    JI Read, O Agertz, MLM Collins (2016)Dark matter cores all the way down, In: Monthly Notices of the Royal Astronomical Society459pp. 2573-2590 Oxford University Press

    We use high resolution simulations of isolated dwarf galaxies to study the physics of dark matter cusp-core transformations at the edge of galaxy formation: M200 = 107 109M .We work at a resolution ( 4 pc minimum cell size; 250M per particle) at which the impact from individual supernovae explosions can be resolved, becoming insensitive to even large changes in our numerical `sub-grid' parameters. We nd that our dwarf galaxies give a remarkable match to the stellar light pro le; star formation history; metallicity distribution function; and star/gas kinematics of isolated dwarf irregular galaxies. Our key result is that dark matter cores of size comparable to the stellar half mass radius r1=2 always form if star formation proceeds for long enough. Cores fully form in less than 4 Gyrs for the M200 = 108M and 14 Gyrs for the 109M dwarf. We provide a convenient two parameter `coreNFW' tting function that captures this dark matter core growth as a function of star formation time and the projected stellar half mass radius. Our results have several implications: (i) we make a strong prediction that if CDM is correct, then `pristine' dark matter cusps will be found either in systems that have truncated star formation and/or at radii r > r1=2; (ii) complete core formation lowers the projected velocity dispersion at r1=2 by a factor 2, which is su cient to fully explain the `too big to fail problem'; and (iii) cored dwarfs will be much more susceptible to tides, leading to a dramatic scouring of the subhalo mass function inside galaxies and groups.

    JI Read, N Trentham (2005)The baryonic mass function of galaxies, In: Royal Society of London Philosophical Transactions Series A363pp. 2693-2693
    D Adén, MI Wilkinson, JI Read, S Feltzing, A Koch, GF Gilmore, EK Grebel, I Lundström (2009)A New Low Mass for the Hercules dSph: The End of a Common Mass Scale for the Dwarfs?, In: _apjl706pp. L150-L154
    T Bruch, AHG Peter, J Read, L Baudis, G Lake (2009)Dark matter disc enhanced neutrino fluxes from the Sun and Earth, In: Physics Letters B674pp. 250-256
    VP Debattista, B Moore, T Quinn, S Kazantzidis, R Maas, L Mayer, J Read, J Stadel (2008)The Causes of Halo Shape Changes Induced by Cooling Baryons: Disks versus Substructures, In: _apj681pp. 1076-1088
    U Ural, MI Wilkinson, JI Read, MG Walker (2015)A low pre-infall mass for the Carina dwarf galaxy from disequilibrium modelling., In: Nat Commun6pp. 7599-7599 Macmillan Publishers Limited.

    Dark matter-only simulations of galaxy formation predict many more subhalos around a Milky Way-like galaxy than the number of observed satellites. Proposed solutions require the satellites to inhabit dark matter halos with masses 10(9)-10(10 )Msun at the time they fell into the Milky Way. Here we use a modelling approach, independent of cosmological simulations, to obtain a pre-infall mass of 3.6(-2.3)(+3.8) × 10(8) Msun for one of the Milky Way's satellites: Carina. This determination of a low halo mass for Carina can be accommodated within the standard model only if galaxy formation becomes stochastic in halos below ∼10(10 )Msun. Otherwise Carina, the eighth most luminous Milky Way dwarf, would be expected to inhabit a significantly more massive halo. The implication of this is that a population of 'dark dwarfs' should orbit the Milky Way: halos devoid of stars and yet more massive than many of their visible counterparts.

    AC Boley, G Lake, J Read, R Teyssier (2009)Globular Cluster Formation Within a Cosmological Context, In: _apjl706pp. L192-L196
    Duncan A Forbes, Justin I Read, Mark Gieles, Michelle L M Collins (2018)Extending the globular cluster system–halo mass relation to the lowest galaxy masses, In: Monthly Notices of the Royal Astronomical Society481(4)pp. 5592-5605

    High-mass galaxies, with halo masses M200 ≥ 1010M⊙ , reveal a remarkable near-linear relation between their globular cluster (GC) system mass and their host galaxy halo mass. Extending this relation to the mass range of dwarf galaxies has been problematic due to the difficulty in measuring independent halo masses. Here we derive new halo masses based on stellar and H i gas kinematics for a sample of nearby dwarf galaxies with GC systems. We find that the GC system mass–halo mass relation for galaxies populated by GCs holds from halo masses of M200 ∼ 1014 M ⊙ ⊙ down to below M200 ∼109 M ⊙ ⊙ , although there is a substantial increase in scatter towards low masses. In particular, three well-studied ultradiffuse galaxies, with dwarf-like stellar masses, reveal a wide range in their GC-to-halo mass ratios. We compare our GC system–halo mass relation to the recent model of El Badry et al., finding that their fiducial model does not reproduce our data in the low-mass regime. This may suggest that GC formation needs to be more efficient than assumed in their model, or it may be due to the onset of stochastic GC occupation in low-mass haloes. Finally, we briefly discuss the stellar mass–halo mass relation for our low-mass galaxies with GCs, and we suggest some nearby dwarf galaxies for which searches for GCs may be fruitful.

    V Henault-Brunet, M Gieles, O Agertz, JI Read (2015)Multiple populations in globular clusters: the distinct kinematic imprints of different formation scenarios, In: MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY450(2)pp. 1164-1198 OXFORD UNIV PRESS
    Filippo Contenta, Eduardo Balbinot, James Petts, Justin Read, Mark Gieles, Michelle Collins, Jorge Peñarrubia, Maxime Delorme, Alessia Gualandris (2018)Probing dark matter with star clusters: a dark matter core in the ultra-faint dwarf Eridanus II, In: Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP)

    We present a new technique to probe the central dark matter (DM) density profile of galaxies that harnesses both the survival and observed properties of star clusters. As a first application, we apply our method to the `ultra-faint' dwarf Eridanus II (Eri II) that has a lone star cluster ~45 pc from its centre. Using a grid of collisional N-body simulations, incorporating the effects of stellar evolution, external tides and dynamical friction, we show that a DM core for Eri II naturally reproduces the size and the projected position of its star cluster. By contrast, a dense cusped galaxy requires the cluster to lie implausibly far from the centre of Eri II (>1 kpc), with a high inclination orbit that must be observed at a particular orbital phase. Our results imply that either a cold DM cusp was `heated up' at the centre of Eri II by bursty star formation, or we are seeing an evidence for physics beyond cold DM.

    T Bruch, J Read, L Baudis, G Lake (2009)Detecting the Milky Way’s Dark Disk, In: _apj696pp. 920-923
    A Pontzen, JI Read, R Teyssier, F Governato, A Gualandris, N Roth, J Devriendt (2015)Milking the spherical cow - on aspherical dynamics in spherical coordinates, In: MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY451(2)pp. 1366-1379 OXFORD UNIV PRESS
    JI Read, L Mayer, AM Brooks, F Governato, G Lake (2009)A dark matter disc in three cosmological simulations of Milky Way mass galaxies, In: _mnras397pp. 44-51
    H Lux, JI Read, G Lake, KV Johnston (2012)NGC 5466: a unique probe of the Galactic halo shape, In: _mnras424pp. L16-L20
    Luca Amendola, Stephen Appleby, Anastasios Avgoustidis, David Bacon, Tessa Baker, Marco Baldi, Nicola Bartolo, Alain Blanchard, Camille Bonvin, Stefano Borgani, Enzo Branchini, Clare Burrage, Stefano Camera, Carmelita Carbone, Luciano Casarini, Mark Cropper, Claudia de Rham, Jörg P Dietrich, Cinzia di Porto, Ruth Durrer, Anne Ealet, Pedro G Ferreira, Fabio Finelli, Juan Garcia-Bellido, Tommaso Giannantonio, Luigi Guzzo, Alan Heavens, Lavinia Heisenberg, Catherine Heymans, Henk Hoekstra, Lukas Hollenstein, Rory Holmes, Zhiqi Hwang, Knud Jahnke, Thomas D Kitching, Tomi Koivisto, Martin Kunz, Giuseppe La Vacca, Eric Linder, Marisa March, Valerio Marra, Carlos Martins, Elisabetta Majerotto, Dida Markovic, David Marsh, Federico Marulli, Richard Massey, Yannick Mellier, Francesco Montanari, David F Mota, Nelson J Nunes, Will Percival, Valeria Pettorino, Cristiano Porciani, Claudia Quercellini, Justin Read, Massimiliano Rinaldi, Domenico Sapone, Ignacy Sawicki, Roberto Scaramella, Constantinos Skordis, Fergus Simpson, Andy Taylor, Shaun Thomas, Roberto Trotta, Licia Verde, Filippo Vernizzi, Adrian Vollmer, Yun Wang, Jochen Weller, Tom Zlosnik (2018)Cosmology and fundamental physics with the Euclid satellite, In: Living Reviews in Relativity Ludwig-Maximilians-Universität München

    Euclid is a European Space Agency medium-class mission selected for launch in 2020 within the cosmic vision 2015-2025 program. The main goal of Euclid is to understand the origin of the accelerated expansion of the universe. Euclid will explore the expansion history of the universe and the evolution of cosmic structures by measuring shapes and red-shifts of galaxies as well as the distribution of clusters of galaxies over a large fraction of the sky. Although the main driver for Euclid is the nature of dark energy, Euclid science covers a vast range of topics, from cosmology to galaxy evolution to planetary research. In this review we focus on cosmology and fundamental physics, with a strong emphasis on science beyond the current standard models. We discuss five broad topics: dark energy and modified gravity, dark matter, initial conditions, basic assumptions and questions of methodology in the data analysis. This review has been planned and carried out within Euclid's Theory Working Group and is meant to provide a guide to the scientific themes that will underlie the activity of the group during the preparation of the Euclid mission.

    Alexander Hobbs, Justin Read, Chris Power, David Cole (2013)Thermal instabilities in cooling galactic coronae: fuelling star formation in galactic discs, In: Monthly notices of the Royal Astronomical Society434(3)pp. 1849-1868 Oxford Univ Press

    We investigate the means by which cold gas can accrete on to Milky Way mass galaxies from a hot corona of gas, using a new smoothed particle hydrodynamics code, 'SPHS'. We find that the 'cold clumps' seen in many classic SPH simulations in the literature are not present in our SPHS simulations. Instead, cold gas condenses from the halo along filaments that form at the intersection of supernovae-driven bubbles from previous phases of star formation. This positive feedback feeds cold gas to the galactic disc directly, fuelling further star formation. The resulting galaxies in the SPH and SPHS simulations differ greatly in their morphology, gas phase diagrams and stellar content. We show that the classic SPH cold clumps owe to a numerical thermal instability caused by an inability for cold gas to mix in the hot halo. The improved treatment of mixing in SPHS suppresses this instability leading to a dramatically different physical outcome. In our highest resolution SPHS simulation, we find that the cold filaments break up into bound, physically motivated clumps that form stars. The filaments are overdense by a factor of 10-100 compared to the surrounding gas, suggesting that the fragmentation results from a physical non-linear instability driven by the overdensity. This 'fragmenting filament' mode of disc growth has important implications for galaxy formation, in particular the role of star formation in bringing cold gas into disc galaxies.

    V. Bonnivard, C. Combet, M. Daniel, S. Funk, A. Geringer-Sameth, J. A. Hinton, D. Maurin, J. I. Read, S. Sarkar, M. G. Walker, M. I. Wilkinson (2015)Dark matter annihilation and decay in dwarf spheroidal galaxies: the classical and ultrafaint dSphs, In: Monthly notices of the Royal Astronomical Society453(1)pp. 849-867 Oxford Univ Press

    Dwarf spheroidal (dSph) galaxies are prime targets for present and future gamma-ray telescopes hunting for indirect signals of particle darkmatter. The interpretation of the data requires careful assessment of their dark matter content in order to derive robust constraints on candidate relic particles. Here, we use an optimized spherical Jeans analysis to reconstruct the 'astrophysical factor' for both annihilating and decaying dark matter in 21 known dSphs. Improvements with respect to previous works are: (i) the use of more flexible luminosity and anisotropy profiles to minimize biases, (ii) the use of weak priors tailored on extensive sets of contamination-free mock data to improve the confidence intervals, (iii) systematic cross-checks of binned and unbinned analyses on mock and real data, and (iv) the use of mock data including stellar contamination to test the impact on reconstructed signals. Our analysis provides updated values for the dark matter content of 8 'classical' and 13 'ultrafaint' dSphs, with the quoted uncertainties directly linked to the sample size; themore flexible parametrizationwe use results in changes compared to previous calculations. This translates into our ranking of potentially-brightest and most robust targets - namely Ursa Minor, Draco, Sculptor - and of the more promising, but uncertain targets -namely Ursa Major 2, Coma - for annihilating dark matter. Our analysis of Segue 1 is extremely sensitive to whether we include or exclude a few marginal member stars, making this target one of the most uncertain. Our analysis illustrates challenges that will need to be addressed when inferring the dark matter content of new 'ultrafaint' satellites that are beginning to be discovered in southern sky surveys.

    Mahmoud Hashim, Amr A. El-Zant, Jonathan Freundlich, Justin Read, Francoise Combes (2023)Halo heating from fluctuating gas in a model dwarf, In: Monthly notices of the Royal Astronomical Society521(1)pp. 772-789 Oxford Univ Press

    The cold dark matter (CDM) structure formation scenario faces challenges on (sub)galactic scales, central among them being the 'cusp-core' problem. A known remedy, driving CDM out of Galactic Centres, invokes interactions with baryons, through fluctuations in the gravitational potential arising from feedback or orbiting clumps of gas or stars. Here, we interpret core formation in a hydrodynamic simulation in terms of a theoretical formulation, which may be considered a generalization of Chandrasekhar's theory of two body relaxation to the case when the density fluctuations do not arise from white noise; it presents a simple characterization of the effects of complex hydrodynamics and 'subgrid physics'. The power spectrum of gaseous fluctuations is found to follow a power law over a range of scales, appropriate for a fully turbulent compressible medium. The potential fluctuations leading to core formation are nearly normally distributed, which allows for the energy transfer leading to core formation to be described as a standard diffusion process, initially increasing the velocity dispersion of test particles as in Chandrasekhar's theory. We calculate the energy transfer from the fluctuating gas to the halo and find it consistent with theoretical expectations. We also examine how the initial kinetic energy input to halo particles is redistributed to form a core. The temporal mass decrease inside the forming core may be fit by an exponential form; a simple prescription based on our model associates the characteristic time-scale with an energy relaxation time. We compare the resulting theoretical density distribution with that in the simulation.

    Martin P. Rey, Andrew Pontzen, Oscar Agertz, Matthew D.A. Orkney, Justin I. Read, Joakim Rosdahl (2020)EDGE: From quiescent to gas-rich to star-forming low-mass dwarf galaxies, In: Mon.Not.Roy.Astron.Soc497(2)pp. 1508-1520

    We study how star formation is regulated in low-mass field dwarf galaxies (⁠|$10^5 \le M_{\star } \le 10^6 \, \mbox{M}_\mathrm{\odot }$|⁠), using cosmological high-resolution (⁠|$3 \, \mathrm{pc}$|⁠) hydrodynamical simulations. Cosmic reionization quenches star formation in all our simulated dwarfs, but three galaxies with final dynamical masses of |$3 \times 10^{9} \, \mbox{M}_\mathrm{\odot }$| are subsequently able to replenish their interstellar medium by slowly accreting gas. Two of these galaxies reignite and sustain star formation until the present day at an average rate of |$10^{-5} \, \mbox{M}_\mathrm{\odot } \, \text{yr}^{-1}$|⁠, highly reminiscent of observed low-mass star-forming dwarf irregulars such as Leo T. The resumption of star formation is delayed by several billion years due to residual feedback from stellar winds and Type Ia supernovae; even at z = 0, the third galaxy remains in a temporary equilibrium with a large gas content but without any ongoing star formation. Using the ‘genetic modification’ approach, we create an alternative mass growth history for this gas-rich quiescent dwarf and show how a small |$(0.2\, \mathrm{dex})$| increase in dynamical mass can overcome residual stellar feedback, reigniting star formation. The interaction between feedback and mass build-up produces a diversity in the stellar ages and gas content of low-mass dwarfs, which will be probed by combining next-generation H i and imaging surveys.

    T. Hilmi, Minchev, T. Buck, M. Martig, A. C. Quillen, G. Monari, B. Famaey, R. S. de Jong, C. F. P. Laporte, J. Read, J. L. Sanders, M. Steinmetz, C. Wegg (2020)Fluctuations in galactic bar parameters due to bar-spiral interaction, In: Monthly notices of the Royal Astronomical Society497(1)pp. 933-955 Oxford Univ Press

    We study the late-time evolution of the central regions of two Milky Way (MW)-like simulations of galaxies formed in a cosmological context, one hosting a fast bar and the other a slow one. We find that bar length, R-b, measurements fluctuate on a dynamical time-scale by up to 100 per cent, depending on the spiral structure strength and measurement threshold. The bar amplitude oscillates by about 15 per cent, correlating with Rb. The Tremaine-Weinberg method estimates of the bars' instantaneous pattern speeds show variations around the mean of up to similar to 20 per cent, typically anticorrelating with the bar length and strength. Through power spectrum analyses, we establish that these bar pulsations, with a period in the range similar to 60-200 Myr, result from its interaction with multiple spiral modes, which are coupled with the bar. Because of the presence of odd spiral modes, the two bar halves typically do not connect at exactly the same time to a spiral arm, and their individual lengths can be significantly offset. We estimated that in about 50 per cent of bar measurements in MW-mass external galaxies, the bar lengths of SBab-type galaxies are overestimated by similar to 15 per cent and those of SBbc types by similar to 55 per cent. Consequently, bars longer than their corotation radius reported in the literature, dubbed 'ultrafast bars', may simply correspond to the largest biases. Given that the Scutum-Centaurus arm is likely connected to the near half of the MW bar, recent direct measurements may be overestimating its length by 1-1.5 kpc, while its present pattern speed may be 5-10 km s(-1) kpc(-1) smaller than its time-averaged value.

    Jonathan P. Coles, Justin I. Read, Prasenjit Saha (2014)Gravitational lens recovery with GLASS: measuring the mass profile and shape of a lens, In: Monthly notices of the Royal Astronomical Society445(3)pp. 2181-2197 Oxford Univ Press

    We use a new non-parametric gravitational modelling tool - GLASS - to determine what quality of data (strong lensing, stellar kinematics, and/or stellar masses) are required to measure the circularly averaged mass profile of a lens and its shape. GLASS uses an underconstrained adaptive grid of mass pixels to model the lens, searching through thousands of models to marginalize over model uncertainties. Our key findings are as follows: (i) for pure lens data, multiple sources with wide redshift separation give the strongest constraints as this breaks the well-known mass-sheet or steepness degeneracy; (ii) a single quad with time delays also performs well, giving a good recovery of both the mass profile and its shape; (iii) stellar masses - for lenses where the stars dominate the central potential - can also break the steepness degeneracy, giving a recovery for doubles almost as good as having a quad with time-delay data, or multiple source redshifts; (iv) stellar kinematics provide a robust measure of the mass at the half-light radius of the stars r(1/2) that can also break the steepness degeneracy if the Einstein radius r(E) not equal r(1/2); and (v) if r(E) similar to r(1/2), then stellar kinematic data can be used to probe the stellar velocity anisotropy beta - an interesting quantity in its own right. Where information on the mass distribution from lensing and/or other probes becomes redundant, this opens up the possibility of using strong lensing to constrain cosmological models.

    Luca Amendola, Stephen Appleby, David Bacon, Tessa Baker, Marco Baldi, Nicola Bartolo, Alain Blanchard, Camille Bonvin, Stefano Borgani, Enzo Branchini, Clare Burrage, Stefano Camera, Carmelita Carbone, Luciano Casarini, Mark Cropper, Claudia deRham, Cinzia di Porto, Anne Ealet, Pedro Ferreira, Fabio Finelli, Juan Garcia-Bellido, Tommaso Giannantonio, Luigi Guzzo, Alan Heavens, Lavinia Heisenberg, Catherine Heymans, Henk Hoekstra, Lukas Hollenstein, Rory Holmes, Ole Horst, Knud Jahnke, Thomas Kitching, Tomi Koivisto, Martin Kunz, Giuseppe La Vacca, Marisa March, Elisabetta Majerotto, Katarina Markovic, David Marsh, Federico Marulli, Richard Massey, Yannick Mellier, David Mota, Nelson Nunes, Will Percival, Valeria Pettorino, Cristiano Porciani, Claudia Quercellini, Justin Read, Massimiliano Rinaldi, Domenico Sapone, Roberto Scaramella, Constantinos Skordis, Fergus Simpson, Andy Taylor, Shaun Thomas, Roberto Trotta, Licia Verde, Filippo Vernizzi, Adrian Vollmer, Yun Wang, Jochen Weller, Tom Zlosnik (2013)Cosmology and fundamental physics with the Euclid satellite, In: arXiv.org16(1)pp. 6-6 Cornell University Library, arXiv.org

    Euclid is a European Space Agency medium class mission selected for launch in 2019 within the Cosmic Vision 2015-2025 programme. The main goal of Euclid is to understand the origin of the accelerated expansion of the Universe. Euclid will explore the expansion history of the Universe and the evolution of cosmic structures by measuring shapes and redshifts of galaxies as well as the distribution of clusters of galaxies over a large fraction of the sky. Although the main driver for Euclid is the nature of dark energy, Euclid science covers a vast range of topics, from cosmology to galaxy evolution to planetary research. In this review we focus on cosmology and fundamental physics, with a strong emphasis on science beyond the current standard models. We discuss five broad topics: dark energy and modified gravity, dark matter, initial conditions, basic assumptions and questions of methodology in the data analysis. This review has been planned and carried out within Euclid's Theory Working Group and is meant to provide a guide to the scientific themes that will underlie the activity of the group during the preparation of the Euclid mission.

    C. Power, J. I. Read, A. Hobbs (2014)The formation of entropy cores in non-radiative galaxy cluster simulations: smoothed particle hydrodynamics versus adaptive mesh refinement, In: Monthly notices of the Royal Astronomical Society440(4)pp. 3243-3256 Oxford Univ Press

    We simulate cosmological galaxy cluster formation using three different approaches to solving the equations of non-radiative hydrodynamics - classic smoothed particle hydrodynamics (SPH), novel SPH with a higher order dissipation switch (SPHS), and an adaptive mesh refinement (AMR) method. Comparing spherically averaged entropy profiles, we find that SPHS and AMR approaches result in a well-defined entropy core that converges rapidly with increasing mass and force resolution. In contrast, the central entropy profile in the SPH approach is sensitive to the cluster's assembly history and shows poor numerical convergence. We trace this disagreement to the known artificial surface tension in SPH that appears at phase boundaries. Varying systematically numerical dissipation in SPHS, we study the contributions of numerical and physical dissipation to the entropy core and argue that numerical dissipation is required to ensure single-valued fluid quantities in converging flows. However, provided it occurs only at the resolution limit and does not propagate errors to larger scales, its effect is benign - there is no requirement to build 'sub-grid' models of unresolved turbulence for galaxy cluster simulations. We conclude that entropy cores in non-radiative galaxy cluster simulations are physical, resulting from entropy generation in shocked gas during cluster assembly.

    H. Lux, J. I. Read, G. Lake, K. V. Johnston (2013)Constraining the Milky Way halo shape using thin streams, In: Monthly notices of the Royal Astronomical Society436(3)pp. 2386-2397 Oxford Univ Press

    Tidal streams are a powerful probe of the Milky Way (MW) potential shape. In this paper, we introduce a simple test-particle method to fit stream data, using a Markov Chain Monte Carlo technique to marginalize over uncertainties in the progenitor's orbit and the MW halo shape parameters. Applying it to mock data of thin streams in the MW halo, we show that, even for very cold streams, stream-orbit offsets - not modelled in our simple method - introduce systematic biases in the recovered shape parameters. For the streams that we consider, and our particular choice of potential parametrization, these errors are of the order of similar to 20 per cent on the halo flattening parameters. However, larger systematic errors can arise for more general streams and potentials; such offsets need to be correctly modelled in order to obtain an unbiased recovery of the underlying potential. Assessing which of the known MW streams are most constraining, we find that NGC 5466 and Pal 5 are the most promising candidates. These form an interesting pair as their orbital planes are both approximately perpendicular to each other and to the disc, giving optimal constraints on the MW halo shape. We show that - while with current data their constraints on potential parameters are poor - good radial velocity data along the Pal 5 stream will provide constraints on q(z) - the flattening perpendicular to the disc. Furthermore, as discussed in a companion paper, NGC 5466 can provide rather strong constraints on the MW halo shape parameters, if the tentative evidence for a departure from the smooth orbit towards its western edge is confirmed.

    Nazim A Bharmal, David F Buscher, Christopher A Haniff, Justin I Read (2003)A novel wavefront sensor for interferometry, In: Proceedings of SPIE4838(1)pp. 721-728

    A new design for a wavefront sensor suitable for low-order low-light correction is shown. The hybrid modal sensor, the Nine Element (NE) sensor, is compared with a curvature sensor and quadcell under single aperture applications. The design of the NE sensor allows the use of readily-available array detectors. We discuss the optimization of the design to maximize its performance with respect to the number of Zernike polynomials to detect and optical parameters, using a simulated annealing technique. Numerical simulations show the good SNR response low-light levels, and indicate a reduction in wavefront variance from 6.41 rad to 2.01 rad . The sensitivity to tip/tilt errors is demonstrated to be comparable to a quadcell. Successful closed feedback loop operation results in corrected Strehl ratios of over 0.5. Improvements and future work is discussed.

    Hamish Silverwood, Sofia Sivertsson, Justin Read, Gianfranco Bertone, Pascal Steger (2017)Determining the Local Dark Matter Density with SDSS G-dwarf data, In: A RecioBlanco, P DeLaverny, AGA Brown, T Prusti (eds.), ASTROMETRY AND ASTROPHYSICS IN THE GAIA SKY12(S330)pp. 255-258 Cambridge Univ Press

    We present a determination of the local dark matter density derived using the integrated Jeans equation method presented in Silverwood et al. (2016) applied to SDSS-SEGUE G-dwarf data processed by Budenbender et al. (2015). For our analysis we construct models for the tracer density, dark matter and baryon distribution, and tilt term (linking radial and vertical motions), and then calculate the vertical velocity dispersion using the integrated Jeans equation. These models are then fit to the data using MULTINEST, and a posterior distribution for the local dark matter density is derived. We find the most reliable determination to come from the alpha-young population presented in Budenbender et al. (2015), yielding a result of rho DM = 0.46(-0.09)(+0.07) GeV cm(-3) = 0.012(-0.002)(+0.001)M(circle dot) pc(-3). Our results also illuminate the path ahead for future analyses using Gaia DR2 data, highlighting which quantities will need to be determined and which assumptions could be relaxed.

    J. A. Petts, A. Gualandris, J. I. Read (2016)A semi-analytic dynamical friction model that reproduces core-stalling (vol 454, pg 3778, 2015), In: Monthly notices of the Royal Astronomical Society460(3)pp. 2337-2338 Oxford Univ Press
    Weiguang Cui, Chris Power, Alexander Knebe, Scott T. Kay, Federico Sembolini, Pascal J. Elahi, Gustavo Yepes, Frazer Pearce, Daniel Cunnama, Alexander M. Beck, Claudio Dalla Vecchia, Romeel Dave, Sean February, Shuiyao Huang, Alex Hobbs, Neal Katz, Ian G. McCarthy, Giuseppe Murante, Valentin Perret, Ewald Puchwein, Justin I. Read, Alexandro Saro, Romain Teyssier, Robert J. Thacker (2016)nIFTy galaxy cluster simulations - IV. Quantifying the influence of baryons on halo properties, In: Monthly notices of the Royal Astronomical Society458(4)pp. 4052-4073 Oxford Univ Press

    Building on the initial results of the nIFTy simulated galaxy cluster comparison, we compare and contrast the impact of baryonic physics with a single massive galaxy cluster, run with 11 state-of-the-art codes, spanning adaptive mesh, moving mesh, classic and modern smoothed particle hydrodynamics (SPH) approaches. For each code represented we have a dark-matter-only (DM) and non-radiative (NR) version of the cluster, as well as a full physics (FP) version for a subset of the codes. We compare both radial mass and kinematic profiles, as well as global measures of the cluster (e.g. concentration, spin, shape), in the NR and FP runs with that in the DM runs. Our analysis reveals good consistency (less than or similar to 20 per cent) between global properties of the cluster predicted by different codes when integrated quantities are measured within the virial radius R-200. However, we see larger differences for quantities within R-2500, especially in the FP runs. The radial profiles reveal a diversity, especially in the cluster centre, between the NR runs, which can be understood straightforwardly from the division of codes into classic SPH and non-classic SPH (including the modern SPH, adaptive and moving mesh codes); and between the FP runs, which can also be understood broadly from the division of codes into those that include active galactic nucleus feedback and those that do not. The variation with respect to the median is much larger in the FP runs with different baryonic physics prescriptions than in the NR runs with different hydrodynamics solvers.

    Gregory R. Ruchti, Justin I. Read, Sofia Feltzing, Antonio Pipino, Thomas Bensby (2014)The hunt for the Milky Way's accreted disc, In: Monthly notices of the Royal Astronomical Society444(1)pp. 515-526 Oxford Univ Press

    The Milky Way is expected to host an accreted disc of stars and dark matter. This forms as massive greater than or similar to 1 : 10 mergers are preferentially dragged towards the disc plane by dynamical friction and then tidally shredded. The accreted disc likely contributes only a tiny fraction of the MilkyWay's thin and thick stellar disc. However, it is interesting because (i) its associated 'dark disc' has important implications for experiments hoping to detect a dark matter particle in the laboratory; and (ii) the presence or absence of such a disc constrains the merger history of our Galaxy. In this work, we develop a chemodynamical template to hunt for the accreted disc. We apply our template to the high-resolution spectroscopic sample from Ruchti et al., finding at present no evidence for accreted disc stars. Our results are consistent with a quiescent Milky Way with no greater than or similar to 1 : 10 mergers since the disc formed and a correspondingly light ` dark disc'. However, we caution that while our method can robustly identify accreted stars, our incomplete stellar sample makes it more challenging to definitively rule them out. Larger unbiased stellar samples will be required for this.

    A. Gatto, F. Fraternali, J. I. Read, F. Marinacci, H. Lux, S. Walch (2013)Unveiling the corona of the Milky Way via ram-pressure stripping of dwarf satellites, In: Monthly notices of the Royal Astronomical Society433(4)pp. 2749-2763 Oxford Univ Press

    The spatial segregation between dwarf spheroidal (dSph) and dwarf irregular galaxies in the Local Group has long been regarded as evidence of an interaction with their host galaxies. In this paper, we assume that ram-pressure stripping is the dominant mechanism that removed gas from the dSphs and we use this to derive a lower bound on the density of the corona of the Milky Way at large distances (R similar to 50-90 kpc) from the Galactic Centre. At the same time, we derive an upper bound by demanding that the interstellar medium of the dSphs is in pressure equilibrium with the hot corona. We consider two dwarfs (Sextans and Carina) with well-determined orbits and star formation histories. Our approach introduces several novel features: (i) we use the measured star formation histories of the dwarfs to derive the time at which they last lost their gas and (via a modified version of the Kennicutt-Schmidt relation) their internal gas density at that time; (ii) we use a large suite of 2D hydrodynamical simulations to model the gas stripping; and (iii) we include supernova feedback tied to the gas content. Despite having very different orbits and star formation histories, we find results for the two dSphs that are in excellent agreement with one another. We derive an average particle density of the corona of the Milky Way at R = 50-90 kpc in the range n(cor) = 1.3-3.6 x 10(-4) cm(-3). Including additional constraints from X-ray emission limits and pulsar dispersion measurements (that strengthen our upper bound), we derive Galactic coronal density profiles. Extrapolating these to large radii, we estimate the fraction of baryons (missing baryons) that can exist within the virial radius of the Milky Way. For an isothermal corona (T-cor = 1.8 x 10(6) K), this is small - just 10-20 per cent of the expected missing baryon fraction, assuming a virial mass of 1-2 x 10(12) M-circle dot. Only a hot (T-cor = 3 x 10(6) K) and adiabatic corona can contain all of the Galaxy's missing baryons. Models for the Milky Way must explain why its corona is in a hot adiabatic thermal state; or why a large fraction of its baryons lie beyond the virial radius.

    Alexander Hobbs, Justin Read, Andrina Nicola (2015)Growing galaxies via superbubble-driven accretion flows, In: Monthly notices of the Royal Astronomical Society452(4)pp. 3593-3609 Oxford Univ Press

    We use a suite of cooling halo simulations to study a new mechanism for rapid accretion of hot halo gas on to star-forming galaxies. Correlated supernova (SN) events create converging 'superbubbles' in the halo gas. Where these collide, the density increases, driving cooling filaments of low-metallicity gas that feed the disc. At our current numerical resolution (similar to 20 pc; m(gas) = 4 x 10(4) M-circle dot) we are only able to resolve the most dramatic events; however, as we increase the numerical resolution, we find that the filaments persist for longer, driving continued late-time star formation. This suggests that SN-driven accretion could act as an efficient mechanism for extracting cold gas from the hot halo, driving late-time star formation in disc galaxies. We show that such filament feeding leads to a peak star formation rate of similar to 3 M-circle dot yr(-1), consistent with estimates for the Milky Way (MW). The filaments we resolve extend to similar to 50 kpc, reaching column densities of N similar to 10(18) cm(-2). We show that such structures can plausibly explain the broad dispersion in Mg II absorption seen along sightlines to quasars. Our results suggest a dual role for stellar feedback in galaxy formation, suppressing hot-mode accretion while promoting cold-mode accretion along filaments. Finally, since the filamentary gas has higher angular momentum than that coming from hot-mode accretion, we show that this leads to the formation of substantially larger gas discs.

    Martin P. Rey, Oscar Agertz, Tjitske K. Starkenburg, Florent Renaud, Gandhali D. Joshi, Andrew Pontzen, Nicolas F. Martin, Diane K. Feuillet, Justin Read (2023)VINTERGATAN-GM: The cosmological imprints of early mergers on Milky-Way-mass galaxies, In: Monthly notices of the Royal Astronomical Society521(1)pp. 995-1012 Oxford Univ Press

    We present a new suite of cosmological zoom-in hydrodynamical ( asymptotic to 20 pc spatial resolution) simulations of Milky-Way mass galaxies to study how a varying mass ratio for a Gaia-Sausage-Enceladus (GSE) progenitor impacts the z = 0 chemodynamics of halo stars. Using the genetic modification approach, we create five cosmological histories for a Milky-Way-mass dark matter halo ( M-200 asymptotic to 10(12) M-?), incrementally increasing the stellar mass ratio of a z asymptotic to 2 merger from 1:25 to 1:2, while fixing the galaxy's final dynamical, stellar mass, and large-scale environment. We find markedly different morphologies at z = 0 following this change in early history, with a growing merger resulting in increasingly compact and bulge-dominated galaxies. Despite this structural diversity, all galaxies show a radially biased population of inner halo stars like the Milky-Way's GSE which, surprisingly, has a similar magnitude, age, [Fe / H], and [ alpha/ Fe] distribution whether the z asymptotic to 2 merger is more minor or major. This arises because a smaller ex-situ population at z asymptotic to 2 is compensated by a larger population formed in an earlier merger-driven starb urst whose contrib ution to the GES can grow dynamically o v er time, and with both populations strongly o v erlapping in the [Fe / H] - [ alpha/ Fe] plane. Our study demonstrates that multiple high-redshift histories can lead to similar z = 0 chemodynamical features in the halo, highlighting the need for additional constraints to distinguish them, and the importance of considering the full spectrum of progenitors when interpreting z = 0 data to reconstruct our Galaxy's past.

    Claudio Bruderer, Justin I. Read, Jonathan P. Coles, Dominik Leier, Emilio E. Falco, Ignacio Ferreras, Prasenjit Saha (2016)Light versus dark in strong-lens galaxies: dark matter haloes that are rounder than their stars, In: Monthly notices of the Royal Astronomical Society456(1)pp. 870-884 Oxford Univ Press

    We measure the projected density profile, shape and alignment of the stellar and dark matter mass distribution in 11 strong-lens galaxies. We find that the projected dark matter density profile - under the assumption of a Chabrier stellar initial mass function - shows significant variation from galaxy to galaxy. Those with an outermost image beyond similar to 10 kpc are very well fit by a projected Navarro-Frenk-White (NFW) profile; those with images within 10 kpc appear to be more concentrated than NFW, as expected if their dark haloes contract due to baryonic cooling. We find that over several half-light radii, the dark matter haloes of these lenses are rounder than their stellar mass distributions. While the haloes are never more elliptical than e(dm) = 0.2, their stars can extend to e(*) > 0.2. Galaxies with high dark matter ellipticity and weak external shear show strong alignment between light and dark; those with strong shear (gamma greater than or similar to 0.1) can be highly misaligned. This is reassuring since isolated misaligned galaxies are expected to be unstable. Our results provide a new constraint on galaxy formation models. For a given cosmology, these must explain the origin of both very round dark matter haloes and misaligned strong-lens systems.

    G. R. Ruchti, J. I. Read, S. Feltzing, A. M. Serenelli, P. Mcmillan, K. Lind, T. Bensby, M. Bergemann, M. Asplund, Antonella Vallenari, Ettore Flaccomio, Elena Pancino, A. J. Korn, A. Recio-Blanco, A. Bayo, G. Carraro, M. T. Costado, Francesco Damiani, U. Heiter, A. Hourihane, P. Jofré, G. Kordopatis, C. Lardo, P. Laverny, L. Monaco, L. Morbidelli, L. Sbordone, C. C. Worley, Simone Zaggia (2015)The Gaia-ESO Survey: a quiescent Milky Way with no significant dark/stellar accreted disc, In: Monthly notices of the Royal Astronomical Society450(3)pp. 2874-2887

    According to our current cosmological model, galaxies like the Milky Way are expected to experience many mergers over their lifetimes. The most massive of the merging galaxies will be dragged towards the disc plane, depositing stars and dark matter into an accreted disc structure. In this work, we utilize the chemodynamical template developed in Ruchti et al. to hunt for accreted stars. We apply the template to a sample of 4675 stars in the third internal data release from the Gaia-ESO Spectroscopic Survey. We find a significant component of accreted halo stars, but find no evidence of an accreted disc component. This suggests that the Milky Way has had a rather quiescent merger history since its disc formed some 8-10 billion years ago and therefore possesses no significant dark matter disc.

    J I Read (2014)The local dark matter density, In: Journal of physics. G, Nuclear and particle physics41(6)pp. 63101-53 IOP Publishing

    I review current efforts to measure the mean density of dark matter near the Sun. This encodes valuable dynamical information about our Galaxy and is also of great importance for 'direct detection' dark matter experiments. I discuss theoretical expectations in our current cosmology; the theory behind mass modelling of the Galaxy; and I show how combining local and global measures probes the shape of the Milky Way dark matter halo and the possible presence of a 'dark disc'. I stress the strengths and weaknesses of different methodologies and highlight the continuing need for detailed tests on mock data-particularly in the light of recently discovered evidence for disequilibria in the Milky Way disc. I collate the latest measurements of ρdm and show that, once the baryonic surface density contribution Σb is normalized across different groups, there is remarkably good agreement. Compiling data from the literature, I estimate Σb = 54.2 ± 4.9 M pc−2, where the dominant source of uncertainty is in the H i gas contribution. Assuming this contribution from the baryons, I highlight several recent measurements of ρdm in order of increasing data complexity and prior, and, correspondingly, decreasing formal error bars. Comparing these measurements with spherical extrapolations from the Milky Way's rotation curve, I show that the Milky Way is consistent with having a spherical dark matter halo at R0 ∼ 8 kpc. The very latest measures of ρdm based on ∼10 000 stars from the Sloan Digital Sky Survey appear to favour little halo flattening at R0, suggesting that the Galaxy has a rather weak dark matter disc, with a correspondingly quiescent merger history. I caution, however, that this result hinges on there being no large systematics that remain to be uncovered in the SDSS data, and on the local baryonic surface density being Σb ∼ 55 M pc−2. I conclude by discussing how the new Gaia satellite will be transformative. We will obtain much tighter constraints on both Σb and ρdm by having accurate 6D phase space data for millions of stars near the Sun. These data will drive us towards fully three dimensional models of our Galactic potential, moving us into the realm of precision measurements of ρdm.

    Emily J. E Charles, Michelle L. M Collins, R. Michael Rich, Justin I Read, Stacy Y Kim, Rodrigo A Ibata, Nicolas F Martin, Scott C Chapman, Eduardo Balbinot, Daniel R Weisz (2023)Andromeda XXV - a dwarf galaxy with a low central dark matter density, In: Monthly Notices of the Royal Astronomical Society521(3)pp. 3527-3539 Oxford University Press

    Andromeda (And) XXV has previously been reported as a dwarf spheroidal galaxy (dSph) with little-to-no dark matter. However, the uncertainties on this result were significant. In this study, we double the number of member stars and re-derive the kinematics and mass of And XXV. We find that And XXV has a systemic velocity of $\nu_\mathrm{r}=-107.7\pm1.0 \mathrm{~km s}^{-1}$ and a velocity dispersion of $\sigma_\nu=4.5\pm1.0\mathrm{~km s}^{-1}$. With this better constrained velocity dispersion, we derive a mass contained within the half-light radius of $M(r< r_\mathrm{h})=6.9^{+3.2}_{-2.8}\times10^6\mathrm{~M}_\odot$. This mass corresponds to a mass-to-light ratio of $\mathrm{[M/L]}_\mathrm{r_\mathrm{h}}=37^{+17}_{-15}\mathrm{~M}_\odot/\mathrm{L}_\odot$, demonstrating, for the first time, that And XXV has an unambiguous dark matter component. We also measure the metallicity of And XXV to be $\mathrm{[Fe/H]}=-1.9\pm0.1$$\mathrm{~}$dex, which is in agreement with previous results. Finally, we extend the analysis of And XXV to include mass modelling using GravSphere. We find that And XXV has a low central dark matter density, $\rho_\mathrm{DM}(150\mathrm{pc})= 2.7^{+1.8}_{-1.6}\times10^7\mathrm{~M}_\odot\mathrm{kpc}^{-3}$, making And XXV a clear outlier when compared to other Local Group (LG) dSphs of the similar stellar mass. In a companion paper, we will explore whether some combination of dark matter cusp-core transformations and/or tides can explain And XXV's low density.

    J. I. Read, GA Mamon, Eugene Vasiliev, L L Watkins, Matthew G Walker, J Penarrubia, MI Wilkinson, W Dehnen, P. Das (2021)Breaking beta: a comparison of mass modelling methods for spherical systems, In: Monthly Notices of the Royal Astronomical Society501(1)pp. 978-993 Oxford University Press

    We apply four different mass modelling methods to a suite of publicly available mock data for spherical stellar systems. We focus on the recovery of the density and velocity anisotropy as a function of radius, either using line-of-sight velocity data only or adding proper motion data. All methods perform well on isotropic and tangentially anisotropic mock data, recovering the density and velocity anisotropy within their 95 per cent confidence intervals over the radial range 0.25 < R/R1/2 < 4, where R1/2 is the half-light radius. However, radially anisotropic mocks are more challenging. For line-of-sight data alone, only methods that use information about the shape of the velocity distribution function are able to break the degeneracy between the density profile and the velocity anisotropy, β, to obtain an unbiased estimate of both. This shape information can be obtained through directly fitting a global phase-space distribution function, by using higher order ‘virial shape parameters’ or by assuming a Gaussian velocity distribution function locally, but projecting it self-consistently along the line of sight. Including proper motion data yields further improvements, and in this case, all methods give a good recovery of both the radial density and velocity anisotropy profiles.

    Matthew D A Orkney, Justin I Read, Oscar Agertz, Andrew Pontzen, Martin P Rey, Alex Goater, Ethan Taylor, Stacy Y Kim, Maxime Delorme (2022)EDGE: the puzzling ellipticity of Eridanus II’s star cluster and its implications for dark matter at the heart of an ultra-faint dwarf, In: Monthly notices of the Royal Astronomical Society515(1)pp. 185-200 Oxford University Press

    ABSTRACT The Eridanus II (EriII) ‘ultra-faint’ dwarf has a large (15 pc) and low-mass (4.3 × 103 M⊙) star cluster (SC) offset from its centre by 23 ± 3 pc in projection. Its size and offset are naturally explained if EriII has a central dark matter core, but such a core may be challenging to explain in a ΛCDM cosmology. In this paper, we revisit the survival and evolution of EriII’s SC, focusing for the first time on its puzzlingly large ellipticity ($0.31^{+0.05}_{-0.06}$). We perform a suite of 960 direct N-body simulations of SCs, orbiting within a range of spherical background potentials fit to ultra-faint dwarf (UFD) galaxy simulations. We find only two scenarios that come close to explaining EriII’s SC. In the first scenario, EriII has a low-density dark matter core (of size ${\sim}70\, \text{pc}$ and density $\lesssim 2\times 10^8\, \text{M}_{\odot }\, \text{kpc}^{-3}$). In this model, the high ellipticity of EriII’s SC is set at birth, with the lack of tidal forces in the core allowing its ellipticity to remain frozen for long times. In the second scenario, EriII’s SC orbits in a partial core, with its high ellipticity owing to its imminent tidal destruction. However, this latter model struggles to reproduce the large size of EriII’s SC, and it predicts substantial tidal tails around EriII’s SC that should have already been seen in the data. This leads us to favour the cored model. We discuss potential caveats to these findings, and the implications of the cored model for galaxy formation and the nature of dark matter.

    MICHELLE LOUISE MILLER COLLINS, Justin I Read (2022)Observational constraints on stellar feedback in dwarf galaxies, In: Nature astronomy6pp. 647-658 Nature Research

    Feedback to the interstellar medium (ISM) from ionising radiation, stellar winds and supernovae is central to regulating star formation in galaxies. Due to their low mass (M * < 10 9 M), dwarf galaxies are particularly susceptible to such processes, making them ideal sites to study the detailed physics of feedback. In this perspective, we summarise the latest observational evidences for feedback from star forming regions and how this drives the formation of 'superbubbles' and galaxy-wide winds. We discuss the important role of external ionising radiation – 'reionisation' – for the smallest galaxies. And, we discuss the observational evidences that this feedback directly impacts galaxy properties such as their star formation histories, metal content, colours, sizes, morphologies and even their inner dark matter densities. We conclude with a look to the future, summarising the key questions that remain unanswered and listing some of the outstanding challenges for galaxy formation theories.

    Imran Tariq Nasim, Alessia Gualandris, Justin I Read, Fabio Antonini, Walter Dehnen, Maxime Delorme (2021)Formation of the largest galactic cores through binary scouring and gravitational wave recoil, In: Monthly notices of the Royal Astronomical Society502(4)pp. 4794-4814

    Massive elliptical galaxies are typically observed to have central cores in their projected radial light profiles. Such cores have long been thought to form through ‘binary scouring’ as supermassive black holes (SMBHs), brought in through mergers, form a hard binary and eject stars from the galactic centre. However, the most massive cores, like the $\sim 3{\, \mathrm{kpc}}$ core in A2261-BCG, remain challenging to explain in this way. In this paper, we run a suite of dry galaxy merger simulations to explore three different scenarios for central core formation in massive elliptical galaxies: ‘binary scouring’, ‘tidal deposition’, and ‘gravitational wave (GW) induced recoil’. Using the griffin code, we self-consistently model the stars, dark matter, and SMBHs in our merging galaxies, following the SMBH dynamics through to the formation of a hard binary. We find that we can only explain the large surface brightness core of A2261-BCG with a combination of a major merger that produces a small $\sim 1{\, \mathrm{kpc}}$ core through binary scouring, followed by the subsequent GW recoil of its SMBH that acts to grow the core size. Key predictions of this scenario are an offset SMBH surrounded by a compact cluster of bound stars and a non-divergent central density profile. We show that the bright ‘knots’ observed in the core region of A2261-BCG are best explained as stalled perturbers resulting from minor mergers, though the brightest may also represent ejected SMBHs surrounded by a stellar cloak of bound stars.

    Michelle L M Collins, Justin I Read, Rodrigo A Ibata, R Michael Rich, Nicolas F Martin, Jorge Peñarrubia, Scott C Chapman, Erik J Tollerud, Daniel R Weisz (2021)Andromeda XXI – a dwarf galaxy in a low-density dark matter halo, In: Monthly notices of the Royal Astronomical Society505(4)pp. 5686-5701 Oxford University Press

    Andromeda XXI (And XXI) has been proposed as a dwarf spheroidal galaxy with a central dark matter density that is lower than expected in the standard $\Lambda$ cold dark matter ($\Lambda$CDM) cosmology. In this work, we present dynamical observations for 77 member stars in this system, more than doubling previous studies to determine whether this galaxy is truly a low-density outlier. We measure a systemic velocity of $v_r=-363.4\pm 1.0{\rm \, km\, s^{-1}}$ and a velocity dispersion of $\sigma _v=6.1^{+1.0}_{-0.9}{\rm \, km\, s^{-1}}$, consistent with previous work and within $1\sigma$ of predictions made using the modified Newtonian dynamics framework. We also measure the metallicity of our member stars from their spectra, finding a mean value of ${\rm [Fe/H]}=-1.7\pm 0.1$ dex. We model the dark matter density profile of And XXI using an improved version of gravsphere, finding a central density of $\rho _{\rm DM}({\rm 150 pc})=2.6_{-1.5}^{+2.4} \times 10^7 \, {\rm M_\odot \, kpc^{-3}}$ at 68 per cent confidence, and a density at two half-light radii of $\rho _{\rm DM}({\rm 1.75 kpc})=0.9_{-0.2}^{+0.3} \times 10^6 \, {\rm M_\odot \, kpc^{-3}}$ at 68 per cent confidence. These are both a factor of${\sim }3\!-\!5$ lower than the densities expected from abundance matching in $\Lambda$CDM. We show that this cannot be explained by ‘dark matter heating’ since And XXI had too little star formation to significantly lower its inner dark matter density, while dark matter heating only acts on the profile inside the half-light radius. However, And XXI’s low density can be accommodated within $\Lambda$CDM if it experienced extreme tidal stripping (losing ${\gt}95{{\ \rm per\ cent}}$ of its mass), or if it inhabits a low-concentration halo on a plunging orbit that experienced repeated tidal shocks.

    Mateo Prgomet, Martin P Rey, Eric P Andersson, Alvaro Segovia Otero, Oscar Agertz, Florent Renaud, Andrew Pontzen, Justin I Read (2022)EDGE: The sensitivity of ultra-faint dwarfs to a metallicity-dependent initial mass function, In: Monthly notices of the Royal Astronomical Society513(2)pp. 2326-2334 Oxford University Press

    ABSTRACT Motivated by the observed bottom-light initial mass function (IMF) in faint dwarfs, we study how a metallicity-dependent IMF affects the feedback budget and observables of an ultra-faint dwarf galaxy. We model the evolution of a low-mass ($\approx 8 \, \times \, 10^{8} \, \rm M_{\odot }$) dark matter halo with cosmological, zoomed hydrodynamical simulations capable of resolving individual supernovae explosions, which we complement with an empirically motivated subgrid prescription for systematic IMF variations. In this framework, at the low gas metallicities typical of faint dwarfs, the IMF of newborn stellar populations becomes top-heavy, increasing the efficiency of supernova and photoionization feedback in regulating star formation. This results in a 100-fold reduction of the final stellar mass of the dwarf compared to a canonical IMF, at fixed dynamical mass. The increase in the feedback budget is none the less met by increased metal production from more numerous massive stars, leading to nearly constant iron content at z = 0. A metallicity-dependent IMF therefore provides a mechanism to produce low-mass ($\rm M_{\star }\sim 10^3 \rm M_{\odot }$), yet enriched ($\rm [Fe/H]\approx -2$) field dwarf galaxies, thus opening a self-consistent avenue to populate the plateau in $\rm [Fe/H]$ at the faintest end of the mass–metallicity relation.

    Martin P. Rey, Andrew Pontzen, Oscar Agertz, Matthew D. A. Orkney, Justin Read, Amelie Saintonge, Stacy Y. Kim, Payel Das (2022)EDGE: What shapes the relationship between H i and stellar observables in faint dwarf galaxies?, In: Monthly notices of the Royal Astronomical Society511(4)pp. 5672-5681 Oxford Univ Press

    We show how the interplay between feedback and mass-growth histories introduces scatter in the relationship between stellar and neutral gas properties of field faint dwarf galaxies (M-*less than or similar to 10(6) M-circle dot). Across a suite of cosmological, high-resolution zoomed simulations, we find that dwarf galaxies of stellar masses 10(5)

    Sofia Sivertsson, J. I. Read, H. Silverwood, Pablo F. de Salas, Khyati Malhan, Axel Widmark, C. F. P. Laporte, S. Garbari, Katherine Freese (2022)Estimating the local dark matter density in a non-axisymmetric wobbling disc, In: Monthly notices of the Royal Astronomical Society511(2)pp. 1977-1991 Oxford University Press

    The density of dark matter near the Sun, ρ DM, ⊙ , is important for experiments hunting for dark matter particles in the laboratory, and for constraining the local shape of the Milky Way’s dark matter halo. Estimates to date have typically assumed that the Milky Way’s stellar disc is axisymmetric and in a steady-state. Yet the Milky Way disc is neither, exhibiting prominent spiral arms and a bar, and vertical and radial oscillations. We assess the impact of these assumptions on determinations of ρ DM, ⊙  by applying a free-form, steady-state, Jeans method to two different  N -body simulations of Milky Way-like galaxies. In one, the galaxy has experienced an ancient major merger, similar to the hypothesized  Gaia –Sausage–Enceladus; in the other, the galaxy is perturbed more recently by the repeated passage and slow merger of a Sagittarius-like dwarf galaxy. We assess the impact of each of the terms in the Jeans–Poisson equations on our ability to correctly extract ρ DM, ⊙  from the simulated data. We find that common approximations employed in the literature – axisymmetry and a locally flat rotation curve – can lead to significant systematic errors of up to a factor ∼1.5 in the recovered surface mass density ∼2 kpc above the disc plane, implying a fractional error on ρ DM, ⊙  of the order of unity. However, once we add in the tilt term and the rotation curve term in our models, we obtain an unbiased estimate of ρ DM, ⊙ , consistent with the true value within our 95 per cent confidence intervals for realistic 20 per cent uncertainties on the baryonic surface density of the disc. Other terms – the axial tilt, 2nd Poisson and time-dependent terms – contribute less than 10 per cent to ρ DM, ⊙  (given current data) and can be safely neglected for now. In the future, as more data become available, these terms will need to be included in the analysis.

    Anna Genina, Justin Read, Azadeh Fattahi, Carlos S. Frenk (2022)Can tides explain the low dark matter density in Fornax?, In: Monthly notices of the Royal Astronomical Society510(2)pp. 2186-2205 Oxford Univ Press

    The low dark matter density in the Fornax dwarf galaxy is often interpreted as being due to the presence of a constant density 'core', but it could also be explained by the effects of Galactic tides. The latter interpretation has been disfavoured because it is apparently inconsistent with the orbital parameters and star formation history of Fornax. We revisit these arguments with the help of the APOSTLE cosmological hydrodynamics simulations. We show that simulated dwarfs with similar properties to Fornax are able to form stars after infall, so that star formation is not necessarily a good tracer of infall time. We also examine the constraints on the pericentre of Fornax and point out that small pericentres (

    Andrew Pontzen, Martin P. Rey, Corentin Cadiou, Oscar Agertz, Romain Teyssier, Justin Read, Matthew D. A. Orkney (2021)EDGE: a new approach to suppressing numerical diffusion in adaptive mesh simulations of galaxy formation, In: Monthly notices of the Royal Astronomical Society501(2)pp. 1755-1765 Oxford Univ Press

    We introduce a new method to mitigate numerical diffusion in adaptive mesh refinement (AMR) simulations of cosmological galaxy formation, and study its impact on a simulated dwarf galaxy as part of the 'EDGE' project. The target galaxy has a maximum circular velocity of 21 km s(-1) but evolves in a region that is moving at up to 90 km s(-1) relative to the hydrodynamic grid. In the absence of any mitigation, diffusion softens the filaments feeding our galaxy. As a result, gas is unphysically held in the circumgalactic medium around the galaxy for 320 Myr, delaying the onset of star formation until cooling and collapse eventually triggers an initial starburst at z = 9. Using genetic modification, we produce 'velocity-zeroed' initial conditions in which the grid-relative streaming is strongly suppressed; by design, the change does not significantly modify the large-scale structure or dark matter accretion history. The resulting simulation recovers a more physical, gradual onset of star formation starting at z = 17. While the final stellar masses are nearly consistent (4.8 x 10(6) M-circle dot and 4.4 x 10(6) M-circle dot for unmodified and velocity-zeroed, respectively), the dynamical and morphological structure of the z = 0 dwarf galaxies are markedly different due to the contrasting histories. Our approach to diffusion suppression is suitable for any AMR zoom cosmological galaxy formation simulations, and is especially recommended for those of small galaxies at high redshift.

    James Alvey, Nashwan Sabti, Victoria Tiki, Diego Blas, Kyrylo Bondarenko, Alexey Boyarsky, Miguel Escudero, Malcolm Fairbairn, Matthew Orkney, Justin Read (2021)New constraints on the mass of fermionic dark matter from dwarf spheroidal galaxies, In: Monthly notices of the Royal Astronomical Society501(1)pp. 1188-1201 Oxford Univ Press

    Dwarf spheroidal galaxies are excellent systems to probe the nature of fermionic dark matter due to their high observed dark matter phase-space density. In this work, we review, revise, and improve upon previous phase-space considerations to obtain lower bounds on the mass of fermionic dark matter particles. The refinement in the results compared to previous works is realized particularly due to a significantly improved Jeans analysis of the galaxies. We discuss two methods to obtain phase-space bounds on the dark matter mass, one model-independent bound based on Pauli's principle, and the other derived from an application of Liouville's theorem. As benchmark examples for the latter case, we derive constraints for thermally decoupled particles and (non-)resonantly produced sterile neutrinos. Using the Pauli principle, we report a model-independent lower bound of m >= 0.18 keV at 68 per cent CL and m >= 0.13 keV at 95 per cent CL. For relativistically decoupled thermal relics, this bound is strengthened to m >= 0.59 keV at 68 per cent CL and m >= 0.41 keV at 95 per cent CL, while for non-resonantly produced sterile neutrinos the constraint is m >= 2.80 keV at 68 per cent CL and m >= 1.74 keV at 95 per cent CL. Finally, the phase-space bounds on resonantly produced sterile neutrinos are compared with complementary limits from X-ray, Lyman alpha, and big bang nucleosynthesis observations.

    Matthew D. A. Orkney, Justin Read, Martin P. Rey, Imran Nasim, Andrew Pontzen, Oscar Agertz, Stacy Y. Kim, Maxime Delorme, Walter Dehnen (2021)EDGE: two routes to dark matter core formation in ultra-faint dwarfs, In: Monthly notices of the Royal Astronomical Society504(3)pp. 3509-3522 Oxford Univ Press

    In the standard Lambda cold dark matter paradigm, pure dark matter simulations predict dwarf galaxies should inhabit dark matter haloes with a centrally diverging density 'cusp'. This is in conflict with observations that typically favour a constant density 'core'. We investigate this 'cusp-core problem' in 'ultra-faint' dwarf galaxies simulated as part of the 'Engineering Dwarfs at Galaxy formation's Edge' project. We find, similarly to previous work, that gravitational potential fluctuations within the central region of the simulated dwarfs kinematically heat the dark matter particles, lowering the dwarfs' central dark matter density. However, these fluctuations are not exclusively caused by gas inflow/outflow, but also by impulsive heating from minor mergers. We use the genetic modification approach on one of our dwarf's initial conditions to show how a delayed assembly history leads to more late minor mergers and, correspondingly, more dark matter heating. This provides a mechanism by which even ultra-faint dwarfs (M-star < 10(5) M-circle dot), in which star formation was fully quenched at high redshift, can have their central dark matter density lowered over time. In contrast, we find that late major mergers can regenerate a central dark matter cusp, if the merging galaxy had sufficiently little star formation. The combination of these effects leads us to predict significant stochasticity in the central dark matter density slopes of the smallest dwarfs, driven by their unique star formation and mass assembly histories.

    Florent Renaud, Oscar Agertz, Eric P Andersson, Justin I Read, Nils Ryde, Thomas Bensby, Martin P Rey, Diane K Feuillet (2021)VINTERGATAN III: how to reset the metallicity of the Milky Way, In: Monthly notices of the Royal Astronomical Society503(4)pp. 5868-5876

    ABSTRACT Using the cosmological zoom simulation VINTERGATAN, we present a new scenario for the onset of star formation at the metal-poor end of the low-[α/Fe] sequence in a Milky Way-like galaxy. In this scenario, the galaxy is fuelled by two distinct gas flows. One is enriched by outflows from massive galaxies, but not the other. While the former feeds the inner galactic region, the latter fuels an outer gas disc, inclined with respect to the main galactic plane, and with a significantly poorer chemical content. The first passage of the last major merger galaxy triggers tidal compression in the outer disc, which increases the gas density and eventually leads to star formation, at a metallicity 0.75 dex lower than the inner galaxy. This forms the first stars of the low-[α/Fe] sequence. These in situ stars have halo-like kinematics, similar to what is observed in the Milky Way, due to the inclination of the outer disc that eventually aligns with the inner one via gravitational torques. We show that this tilting disc scenario is likely to be common in Milky Way-like galaxies. This process implies that the low-[α/Fe] sequence is populated in situ, simultaneously from two formation channels, in the inner and the outer galaxy, with distinct metallicities. This contrasts with purely sequential scenarios for the assembly of the Milky Way disc and could be tested observationally.

    Florent Renaud, Oscar Agertz, Justin I Read, Nils Ryde, Eric P Andersson, Thomas Bensby, Martin P Rey, Diane K Feuillet (2021)VINTERGATAN – II. The history of the Milky Way told by its mergers, In: Monthly notices of the Royal Astronomical Society503(4)pp. 5846-5867

    ABSTRACT Using the VINTERGATAN cosmological zoom simulation, we explore the contributions of the in situ and accreted material, and the effect of galaxy interactions and mergers in the assembly of a Milky Way-like galaxy. We find that the initial growth phase of galaxy evolution, dominated by repeated major mergers, provides the necessary physical conditions for the assembly of a thick, kinematically hot disc populated by high-[α/Fe] stars, formed both in situ and in accreted satellite galaxies. We find that the diversity of evolutionary tracks followed by the simulated galaxy and its progenitors leads to very little overlap of the in situ and accreted populations for any given chemical composition. At a given age, the spread in [α/Fe] abundance ratio results from the diversity of physical conditions in VINTERGATAN and its satellites, with an enhancement in [α/Fe] found in stars formed during starburst episodes. Later, the cessation of the merger activity promotes the in situ formation of stars in the low-[α/Fe] regime, in a radially extended, thin and overall kinematically colder disc, thus establishing chemically bimodal thin and thick discs, in line with observations. We draw links between notable features in the [Fe/H]-[α/Fe] plane with their physical causes, and propose a comprehensive formation scenario explaining self-consistently, in the cosmological context, the main observed properties of the Milky Way.

    A Genina, J I Read, C S Frenk, S Cole, A Benítez-Llambay, A D Ludlow, J F Navarro, K A Oman, A Robertson (2020)To β or not to β: can higher order Jeans analysis break the mass–anisotropy degeneracy in simulated dwarfs?, In: Monthly notices of the Royal Astronomical Society498(1)pp. 144-163

    ABSTRACT We test a non-parametric higher order Jeans analysis method, GravSphere, on 32 simulated dwarf galaxies comparable to classical Local Group dwarfs like Fornax. The galaxies are selected from A Project Of Simulating The Local Environment (APOSTLE) suite of cosmological hydrodynamics simulations with cold dark matter (CDM) and self-interacting dark matter (SIDM) models, allowing us to investigate cusps and cores in density distributions. We find that, for CDM dwarfs, the recovered enclosed mass profiles have a bias of no more than 10 per cent, with a 50 per cent scatter in the inner regions and a 20 per cent scatter near the half-light radius, consistent with standard mass estimators. The density profiles are also recovered with a bias of no more than 10 per cent and a scatter of 30 per cent in the inner regions. For SIDM dwarfs, the mass and density profiles are recovered within our 95 per cent confidence intervals but are biased towards cuspy dark matter distributions. This is mainly due to a lack of sufficient constraints from the data. We explore the sources of scatter in the accuracy of the recovered profiles and suggest a χ2 statistic to separate successful models from biased ones. Finally, we show that the uncertainties on the mass profiles obtained with GravSphere are smaller than those for comparable Jeans methods and that they can be further improved if stronger priors, motivated by cosmological simulations, are placed on the velocity anisotropy. We conclude that GravSphere is a promising Jeans-based approach for modelling dark matter distributions in dwarf galaxies.

    Alexandre Alvarez, Francesca Calore, Anna Genina, Justin Read, Pasquale Dario Serpico, Bryan Zaldivar (2020)Dark matter constraints from dwarf galaxies with data-driven J-factors, In: Journal of cosmology and astroparticle physics2020(9)pp. 4-4 Iop Publishing Ltd

    We present an updated analysis of the gamma-ray flux from the directions of classical dwarf spheroidal galaxies, deriving new constraints on WIMP dark matter (DM) annihilation using a decade of Fermi-LAT data. Among the major novelties, we infer the dwarfs' J-factors by including new observations without imposing any a priori parametric profile for the DM distribution. While statistically compatible with results obtained from more conventional parameterisations, this procedure reduces the theoretical bias imposed on the data. Furthermore, we retain the full data-driven shape of the J-factors' empirical probability distributions when setting limits on DM, without imposing log-normality as is typically done. In conjunction with the data-driven J-factors, we improve on a new method for estimating the probability distribution function of the astrophysical background at the dwarf position [1], fully profiling over background uncertainties. We show that, for most "classical" dwarfs, the background systematic uncertainty dominates over the uncertainty on their J-factors. Raw distributions of J- and D-factors (the latter being the analogous of J-factors for decaying DM) are available upon request.

    Imran Nasim, Alessia Gualandris, Justin Read, Walter Dehnen, Maxime Delorme, Fabio Antonini (2020)Defeating stochasticity: coalescence time-scales of massive black holes in galaxy mergers, In: Monthly notices of the Royal Astronomical Society497(1)pp. 739-746 Oxford Univ Press

    The coalescence of massive black hole binaries (BHBs) in galactic mergers is the primary source of gravitational waves (GWs) at low frequencies. Current estimates of GW detection rates for the Laser Interferometer Space Antenna and the Pulsar Timing Array vary by three orders of magnitude. To understand this variation, we simulate the merger of equal-mass, eccentric, galaxy pairs with central massive black holes and shallow inner density cusps. We model the formation and hardening of a central BHB using the fast multiple method as a force solver, which features a O(N) scaling with the number N of particles and obtains results equivalent to direct-summation simulations. At N similar to 5 x 10(5), typical for contemporary studies, the eccentricity of the BHBs can vary significantly for different random realizations of the same initial condition, resulting in a substantial variation of the merger time-scale. This scatter owes to the stochasticity of stellar encounters with the BHB and decreases with increasing N. We estimate that N similar to 10(7) within the stellar half-light radius suffices to reduce the scatter in the merger time-scale to similar to 10 per cent. Our results suggest that at least some of the uncertainty in low-frequency GW rates owes to insufficient numerical resolution.

    Oscar Agertz, Florent Renaud, Sofia Feltzing, Justin I Read, Nils Ryde, Eric P Andersson, Martin P Rey, Thomas Bensby, Diane K Feuillet (2021)VINTERGATAN – I. The origins of chemically, kinematically, and structurally distinct discs in a simulated Milky Way-mass galaxy, In: Monthly notices of the Royal Astronomical Society503(4)pp. 5826-5845

    ABSTRACT Spectroscopic surveys of the Milky Way’s stars have revealed spatial, chemical, and kinematical structures that encode its history. In this work, we study their origins using a cosmological zoom simulation, VINTERGATAN, of a Milky Way-mass disc galaxy. We find that in connection to the last major merger at z ∼ 1.5, cosmological accretion leads to the rapid formation of an outer, metal-poor, low-[α/Fe] gas disc around the inner, metal-rich galaxy containing the old high-[α/Fe] stars. This event leads to a bimodality in [α/Fe] over a range of [Fe/H]. A detailed analysis of how the galaxy evolves since z ∼ 1 is presented. We demonstrate the way in which inside-out growth shapes the radial surface density and metallicity profile and how radial migration preferentially relocates stars from the inner disc to the outer disc. Secular disc heating is found to give rise to increasing velocity dispersions and scale heights with stellar age, which together with disc flaring explains several trends observed in the Milky Way, including shallower radial [Fe/H] profiles above the mid-plane. We show how the galaxy formation scenario imprints non-trivial mappings between structural associations (i.e. thick and thin discs), velocity dispersions, α-enhancements, and ages of stars; e.g. the most metal-poor stars in the low-[α/Fe] sequence are found to have a scale height comparable to old high-[α/Fe] stars. Finally, we illustrate how at low spatial resolution, comparable to the thickness of the galaxy, the proposed pathway to distinct sequences in [α/Fe]–[Fe/H] cannot be captured.

    Firat Toguz, Daisuke Kawata, George Seabroke, Justin I Read (2022)Constraining ultra light dark matter with the Galactic nuclear star cluster, In: Monthly notices of the Royal Astronomical Society511(2)pp. 1757-1770 OUP

    ABSTRACT We use the Milky Way’s nuclear star cluster (NSC) to test the existence of a dark matter ‘soliton core’, as predicted in ultra-light dark matter (ULDM) models. Since the soliton core size is proportional to $m_{\rm DM}^{-1}$, while the core density grows as $m_{\rm DM}^{2}$, the NSC (dominant stellar component within ∼3 pc) is sensitive to a specific window in the dark matter particle mass, mDM. We apply a spherical isotropic Jeans model to fit the NSC line-of-sight velocity dispersion data, assuming priors on the precisely measured Milky Way’s supermassive black hole (SMBH) mass and the well-measured NSC density profile. We find that the current observational data reject the existence of a soliton core for a single ULDM particle with mass in the range 10−20.4 eV ≲ mDM ≲ 10−18.5 eV, assuming that the soliton core structure is not affected by the Milky Way’s SMBH. We test our methodology on mock data, confirming that we are sensitive to the same range in ULDM mass as for the real data. Dynamical modelling of a larger region of the Galactic centre, including the nuclear stellar disc, promises tighter constraints over a broader range of mDM. We will consider this in future work.

    H Lux, JI Read, G Lake (2010)Determining Orbits for the Milky Way’s Dwarfs, In: VP Debattista, CC Popescu (eds.), American Institute of Physics Conference Series1240pp. 415-416
    S Garbari, G Lake, J Read (2010)Measuring the Local Dark Matter Density, In: VP Debattista, CC Popescu (eds.), American Institute of Physics Conference Series1240pp. 411-412
    MI Wilkinson, JT Kleyna, NW Evans, GF Gilmore, EK Grebel, A Koch, J Read, R Young (2005)Substructure in dwarf spheroidals - a star cluster connection?, In: H Jerjen, B Binggeli (eds.), IAU Colloq. 198: Near-fields cosmology with dwarf elliptical galaxiespp. 240-243
    S Garbari, JI Read, G Lake (2011)Limits on the local density of dark matter, In: ArXiv e-prints
    J Read (2007)Parameterized equations of state for neutron stars, In: APS Meeting Abstractspp. 11005-11005
    JI Read, MI Wilkinson, NW Evans, G Gilmore, JT Kleyna (2005)The mass of dwarf spheroidal galaxies and the missing satellite problem, In: H Jerjen, B Binggeli (eds.), IAU Colloq. 198: Near-fields cosmology with dwarf elliptical galaxiespp. 235-239
    L Amendola, S Appleby, D Bacon, T Baker, M Baldi, N Bartolo, A Blanchard, C Bonvin, S Borgani, E Branchini, C Burrage, S Camera, C Carbone, L Casarini, M Cropper, C deRham, C di Porto, A Ealet, PG Ferreira, F Finelli, J Garcia-Bellido, T Giannantonio, L Guzzo, A Heavens, L Heisenberg, C Heymans, H Hoekstra, L Hollenstein, R Holmes, O Horst, K Jahnke, TD Kitching, T Koivisto, M Kunz, G La Vacca, M March, E Majerotto, K Markovic, D Marsh, F Marulli, R Massey, Y Mellier, DF Mota, N Nunes, W Percival, V Pettorino, C Porciani, C Quercellini, J Read, M Rinaldi, D Sapone, R Scaramella, C Skordis, F Simpson, A Taylor, S Thomas, R Trotta, L Verde, F Vernizzi, A Vollmer, Y Wang, J Weller, T Zlosnik (2012)Cosmology and fundamental physics with the Euclid satellite, In: ArXiv e-prints
    MI Wilkinson, JT Kleyna, N Wyn Evans, GF Gilmore, JI Read, A Koch, EK Grebel, MJ Irwin (2006)The internal kinematics of dwarf spheroidal galaxies, In: GA Mamon, F Combes, C Deffayet, B Fort (eds.), EAS Publications Series20pp. 105-112
    NW Evans, MI Wilkinson, JT Kleyna, JI Read, G Gilmore (2005)Kinematics and M/L ratios of dwarf spheroidals, In: H Jerjen, B Binggeli (eds.), IAU Colloq. 198: Near-fields cosmology with dwarf elliptical galaxiespp. 60-67
    A Gatto, F Fraternali, JI Read, F Marinacci, H Lux, S Walch (2013)Unveiling the corona of the Milky Way via ram-pressure stripping of dwarf satellites, In: ArXiv e-prints
    S Garbari, JI Read, G Lake (2012)Limits on the local dark matter density, In: European Physical Journal Web of Conferences19pp. 1008-1008
    T Bruch, J Read, L Baudis, G Lake (2008)Signatures of the Milky Way’s dark disk in current and future experiments, In: Identification of Dark Matter 2008
    A Charbonnier, C Combet, M Daniel, S Funk, JA Hinton, D Maurin, C Power, JI Read, S Sarkar, MG Walker, MI Wilkinson (2012)Dark matter in dSph galaxies (Charbonnier+, 2011), In: VizieR Online Data Catalog741pp. 81526-81526
    NA Bharmal, DF Buscher, CA Haniff, JI Read (2003)A novel wavefront sensor for interferometry, In: WA Traub (eds.), Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series4838pp. 721-728

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