Justin Read

Professor Justin Read

Head of the Department of Physics
+44 (0)1483 686814
07 BB 03
Head of Department Enquiries - physicshod@surrey.ac.uk
PA: Mrs Joanna Moore
+44 (0)1483 689421

Academic and research departments

Astrophysics Research Group.


Prof. Justin Read is Head of Physics at the University of Surrey. His main area of research is gravitational probes of dark matter, studying everything from the tiniest galaxies in the Universe, where we can measure how dark matter clusters on the smallest scales, to giant clusters of galaxies, where we can produce images of the distribution of dark matter using gravitational lensing.

Prof. Read completed his PhD in theoretical astrophysics at Cambridge University, UK in 2004. After a two-year postdoctoral research position, also in Cambridge, he moved to the University of Zürich to join the computational science group. In 2009, he joined the University of Leicester as a lecturer in theoretical astrophysics, and in October 2010 he was awarded an SNF assistant professorship at the ETH Zürich. In April 2013, he took up a full Chair at the University of Surrey. Prof. Read was awarded the 2013 MERAC Prize by the European Astronomical Society for his high impact research in computational astrophysics and cosmology. He is a fellow of the Institute of Physics and the Royal Physiographic Society of Lund.

A full list of his publications (freely available for download) is available on arXiv.org and through Prof. Read's Google Scholar profile.

Areas of specialism

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

University roles and responsibilities

  • Head of the Department of Physics


    Research interests

    My teaching

    My publications


    Bharmal NA, Buscher DF, Haniff CA, Read JI (2003) A novel wavefront sensor for interferometry, Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series4838pp. 721-728-721-728
    Hobbs A, Read J, Power C, Cole D (2012) Thermal instabilities in cooling galactic coronae: fuelling star formation in galactic discs, ArXiv e-prints
    Garbari S, Lake G, Read J (2010) Measuring the Local Dark Matter Density, American Institute of Physics Conference Series1240pp. 411-412-411-412
    Tanvir NR, Mackey AD, Ferguson AMN, Huxor A, Read JI, Lewis GF, Irwin MJ, Chapman S, Ibata R, Wilkinson MI, McConnachie AW, Martin NF, Davies MB, Bridges TJ (2012) The structure of star clusters in the outer halo of M31,\mnras422(1)pp. 162-184
    We present a structural analysis of halo star clusters in M31 based on deep Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) imaging. The clusters in our sample span a range in galactocentric projected distance from 13 to 100kpc and thus reside in rather remote environments. Ten of the clusters are classical globulars, whilst four are from the Huxor et al. population of extended, old clusters. For most clusters, contamination by M31 halo stars is slight, and so the profiles can be mapped reliably to large radial distances from their centres. We find that the extended clusters are well fit by analytic King profiles with <20 parsec core radii and <100 parsec photometric tidal radii, or by Sérsic profiles of index <1 (i.e. approximately exponential). Most of the classical globulars also have large photometric tidal radii in the range 50-100 parsec; however, the King profile is a less good fit in some cases, particularly at small radii. We find 60 percent of the classical globular clusters exhibit cuspy cores which are reasonably well described by Sérsic profiles of index <2-6. Our analysis also reinforces the finding that luminous classical globulars, with half-light radii <10 parsec, are present out to radii of at least 100kpc in M31, which is in contrast to the situation in the Milky Way where such clusters (other than the unusual object NGC 2419) are absent beyond 40kpc. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.
    Read JI, Lake G, Agertz P, Debattista V (2008) A dark disc in the Milky Way, Astronomische Nachrichten329(9-10)pp. 1022-1024
    Predicting the flux of dark matter particles through the Earth is vital for current and future direct dark matter detection experiments. To date, such predictions have been based on simulations that model the dark matter alone. Here we make the first attempt to include the influence of the baryonic matter. We show that the presence of a stellar/gas disc at high redshift (z < 1) causes merging satellites to be preferentially dragged towards the disc plane. This results in an accreted stellar disc, and a dark matter disc that contributes PDDISO = 0.25-1 ÁHALO at the solar position. Although not likely to be dynamically interesting, the dark disc has important implications for the direct detection of dark matter because of its low velocity with respect to the Earth. © 2008 WILEY-VCH Verlag GmbH & Co. KGaA.
    Ural U, Wilkinson MI, Read JI, Walker MG (2015) A low pre-infall mass for the Carina dwarf galaxy from disequilibrium modelling.,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.
    Read JI, Hayfield T (2012) SPHS: Smoothed particle hydrodynamics with a higher order dissipation switch, Monthly Notices of the Royal Astronomical Society422(4)pp. 3037-3055
    We present a novel implementation of smoothed particle hydrodynamics that uses the spatial derivative of the velocity divergence as a higher order dissipation switch. Our switch - which is second order accurate - detects flow convergence before it occurs. If particle trajectories are going to cross, we switch on the usual SPH artificial viscosity, as well as conservative dissipation in all advected fluid quantities (e.g. the entropy). The viscosity and dissipation terms (that are numerical errors) are designed to ensure that all fluid quantities remain single valued as particles approach one another, to respect conservation laws, and to vanish on a given physical scale as the resolution is increased. SPHS alleviates a number of known problems with 'classic' SPH, successfully resolving mixing, and recovering numerical convergence with increasing resolution. An additional key advantage is that - treating the particle mass similarly to the entropy - we are able to use multimass particles, giving significantly improved control over the refinement strategy. We present a wide range of code tests including the Sod shock tube, Sedov-Taylor blast wave, Kelvin-Helmholtz Instability, the 'blob test' and some convergence tests. Our method performs well on all tests, giving good agreement with analytic expectations. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.
    Charbonnier A, Combet C, Daniel M, Funk S, Hinton JA, Maurin D, Power C, Read JI, Sarkar S, Walker MG, Wilkinson MI (2012) Dark matter in dSph galaxies (Charbonnier+, 2011), VizieR Online Data Catalog741pp. 81526-81526
    Read JI, Pontzen AP, Viel M (2006) On the formation of dwarf galaxies and stellar haloes, \mnras371pp. 885-897-885-897
    Read JI, Saha P, Macciò AV (2007) Radial Density Profiles of Time-Delay Lensing Galaxies, \apj667pp. 645-654-645-654
    Lux H, Read JI, Lake G, Johnston KV (2012) NGC5466: A unique probe of the galactic halo shape, Monthly Notices of the Royal Astronomical Society: Letters424(1)
    Stellar streams provide unique probes of galactic potentials, with the longer streams normally providing the cleaner measurements. In this Letter, we show an example of a short tidal stream that is particularly sensitive to the shape of the Milky Way's dark matter halo: the globular cluster tidal stream NGC5466. This stream has an interesting deviation from a smooth orbit at its western edge. We show that such a deviation favours an underlying oblate or triaxial halo (irrespective of plausible variations in the MilkyWay disc properties and the specific halo parametrization chosen); spherical or prolate halo shapes can be excluded at a high confidence level. Therefore, more extensive data sets along the NGC5466 tidal stream promise strong constraints on the Milky Way halo shape. © 2012 The Authors, MNRAS 424, L16-L20 Monthly Notices of the Royal Astronomical Society © 2012 RAS.
    Saha P, Read JI, Williams LLR (2006) Two Strong-Lensing Clusters Confront Universal Dark Matter Profiles, \apjl652pp. L5-L8-L5-L8
    Read JI (2004) Dwarf spheroidal galaxies - the key to unlocking the nature of dark matter?, The Observatory124pp. 227-228-227-228
    Goerdt T, Moore B, Read JI, Stadel J, Zemp M (2006) Does the Fornax dwarf spheroidal have a central cusp or core?, \mnras368pp. 1073-1077-1073-1077
    Read J (2007) Parameterized equations of state for neutron stars, APS Meeting Abstractspp. 11005-11005
    Lux H, Read JI, Lake G (2010) Determining orbits for the Milky Way?s dwarfs,\mnras406(4)pp. 2312-2324 - 2312-2324
    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. © 2010 The Authors. Journal compilation © 2010 RAS.
    Read JI, Wilkinson MI, Evans NW, Gilmore G, Kleyna JT (2005) The mass of dwarf spheroidal galaxies and the missing satellite problem, IAU Colloq. 198: Near-fields cosmology with dwarf elliptical galaxiespp. 235-239-235-239
    Garbari S, Read JI, Lake G (2011) Limits on the local dark matter density,\mnras416(3)pp. 2318-2340 - 2318-2340
    We revisit systematics in determining the local dark matter density Ádm from the vertical motion of stars in the solar neighbourhood. Using a simulation of a Milky Way like galaxy, we determine the data quality required to detect Ádm at its expected local value. We introduce a new method for recovering Ádm that uses moments of the Jeans equations, combined with a Markov chain Monte Carlo technique, to marginalize over the unknown parameters. Given sufficiently good data, we show that our method can recover the correct local dark matter density even in the face of disc inhomogeneities, non-isothermal tracers and a non-separable distribution function. We illustrate the power of our technique by applying it to Hipparcos data. We first make the assumption that the A- and F-star tracer populations are isothermal. This recovers Ádm= 0.003+0.009- 0.007M™pc-3 (Ádm= 0.11+0.34- 0.27GeVcm-3, with 90per cent confidence), consistent with previous determinations. However, the vertical dispersion profile of these tracers is poorly known. If we assume instead a non-isothermal profile similar to that of the blue disc stars from SDSS DR-7 recently measured, we obtain a fit with a very similar Ç2 value, but with Ádm= 0.033+0.008- 0.009M™pc-3 (Ádm= 1.25+0.30- 0.34GeVcm-3 with 90per cent confidence). This highlights that it is vital to measure the vertical dispersion profile of the tracers to recover an unbiased estimate of Ádm. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.
    Read JI, Hayfield T, Agertz O (2010) Resolving mixing in smoothed particle hydrodynamics, Monthly Notices of the Royal Astronomical Society405(3)pp. 1513-1530
    Standard formulations of smoothed particle hydrodynamics (SPH) are unable to resolve mixing at fluid boundaries. We use an error and stability analysis of the generalized SPH equations of motion to prove that this is due to two distinct problems. The first is a leading order error in the momentum equation. This should decrease with an increasing neighbour number, but does not because numerical instabilities cause the kernel to be irregularly sampled. We identify two important instabilities: the clumping instability and the banding instability, and we show that both are cured by a suitable choice of kernel. The second problem is the local mixing instability (LMI). This occurs as particles attempt to mix on the kernel scale, but are unable to due to entropy conservation. The result is a pressure discontinuity at boundaries that pushes fluids of different entropies apart. We cure the LMI by using a weighted density estimate that ensures that pressures are single-valued throughout the flow. This also gives a better volume estimate for the particles, reducing errors in the continuity and momentum equations. We demonstrate mixing in our new optimized smoothed particle hydrodynamics (OSPH) scheme using a Kelvin-Helmholtz instability (KHI) test with a density contrast of 1:2, and the 'blob test'- a 1:10 density ratio gas sphere in a wind tunnel - finding excellent agreement between OSPH and Eulerian codes. © 2010 The Authors. Journal compilation © 2010 RAS.
    Garbari S, Read JI, Lake G (2011) Limits on the local density of dark matter, ArXiv e-prints
    Teyssier R, Pontzen A, Dubois Y, Read JI (2013) Cusp-core transformations in dwarf galaxies: Observational predictions, Monthly Notices of the Royal Astronomical Society429(4)pp. 3068-3078
    The presence of a dark matter core in the central kiloparsec of many dwarf galaxies has been a long-standing problem in galaxy formation theories based on the standard cold dark matter paradigm. Recent simulations, based on smooth particle hydrodynamics and rather strong feedback recipes, have shown that it was indeed possible to form extended dark matter cores using baryonic processes related to a more realistic treatment of the interstellar medium. Using adaptive mesh refinement, together with a new, stronger supernova feedback scheme that we have recently implemented in the RAMSES code, we show that it is also possible to form a prominent dark matter core within the well-controlled framework of an isolated, initially cuspy, 1010M™ dark matter halo. Although our numerical experiment is idealized, it allows a clean and unambiguous identification of the dark matter core formation process. Our dark matter inner profile is well fitted by a pseudo-isothermal profile with a core radius of 800 pc. The core formation mechanism is consistent with the one proposed by Pontzen & Governato. We highlight two key observational predictions of all simulations that find cuspcore transformations: (i) a bursty star formation history with a peak-to-trough ratio of 5 to 10 and a duty cycle comparable to the local dynamical time and (ii) a stellar distribution that is hot with v/à ~ 1. We compare the observational properties of our model galaxy with recent measurements of the isolated dwarfWolf-Lundmark-Mellote (WLM).We show that the spatial and kinematical distribution of stars and HI gas are in striking agreement with observations, supporting the fundamental role played by stellar feedback in shaping both the stellar and dark matter distribution. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.
    Read JI, Bruch T, Baudis L, Debattista VP, Agertz O, Mayer L, Brooks AM, Governato F, Peter AHG, Lake G (2010) A Dark Matter Disc in the Milky Way, American Institute of Physics Conference Series1240pp. 391-394-391-394
    Garbari S, Liu C, Read JI, Lake G (2012) A new determination of the local dark matter density from the kinematics of K dwarfs, Monthly Notices of the Royal Astronomical Society425(2)pp. 1445-1458
    We apply a new method to determine the local disc matter and dark halo matter density to kinematic and position data for <2000K dwarf stars taken from the literature. Our method assumes only that the disc is locally in dynamical equilibrium, and that the 'tilt' term in the Jeans equations is small up to <1kpc above the plane. We present a new calculation of the photometric distances to the K dwarf stars, and use a Monte Carlo Markov chain to marginalize over uncertainties in both the baryonic mass distribution, and the velocity and distance errors for each individual star. We perform a series of tests to demonstrate that our results are insensitive to plausible systematic errors in our distance calibration, and we show that our method recovers the correct answer from a dynamically evolved N-body simulation of the Milky Way. We find a local dark matter density of Á dm =0.025-0.013+0.014M ™pc -3 (0.95-0.49+0.53GeVcm -3) at 90per cent confidence assuming no correction for the non-flatness of the local rotation curve, and Á dm =0.022-0.013+0.015M ™pc -3 (0.85-0.50+0.57GeVcm -3) if the correction is included. Our 90per cent lower bound on Á dm is larger than the canonical value typically assumed in the literature, and is at mild tension with extrapolations from the rotation curve that assume a spherical halo. Our result can be explained by a larger normalization for the local Milky Way rotation curve, an oblate dark matter halo, a local disc of dark matter or some combination of these. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.
    Cole DR, Dehnen W, Read JI, Wilkinson MI (2012) The mass distribution of the Fornax dSph: Constraints from its globular cluster distribution, Monthly Notices of the Royal Astronomical Society426(1)pp. 601-613
    Uniquely among the dwarf spheroidal (dSph) satellite galaxies of the Milky Way, Fornax hosts globular clusters. It remains a puzzle as to why dynamical friction has not yet dragged any of Fornax's five globular clusters to the centre, and also why there is no evidence that any similar star cluster has been in the past (for Fornax or any other tidally undisrupted dSph). We set up a suite of 2800 N-body simulations that sample the full range of globular cluster orbits and mass models consistent with all existing observational constraints for Fornax. In agreement with previous work, we find that if Fornax has a large dark matter core, then its globular clusters remain close to their currently observed locations for long times. Furthermore, we find previously unreported behaviour for clusters that start inside the core region. These are pushed out of the core and gain orbital energy, a process we call 'dynamical buoyancy'. Thus, a cored mass distribution in Fornax will naturally lead to a shell-like globular cluster distribution near the core radius, independent of the initial conditions. By contrast, cold dark matter-type cusped mass distributions lead to the rapid infall of at least one cluster within ”t = 1-2Gyr, except when picking unlikely initial conditions for the cluster orbits (<2 per cent probability), and almost all clusters within ”t = 10Gyr. Alternatively, if Fornax has only a weakly cusped mass distribution, then dynamical friction is much reduced. While over ”t = 10Gyr this still leads to the infall of one to four clusters from their present orbits, the infall of any cluster within ”t = 1-2Gyr is much less likely (with probability 0-70 per cent, depending on ”t and the strength of the cusp). Such a solution to the timing problem requires (in addition to a shallow dark matter cusp) that in the past the globular clusters were somewhat further from Fornax than today; they most likely did not form within Fornax, but were accreted. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.
    Boley AC, Lake G, Read J, Teyssier R (2009) Globular Cluster Formation Within a Cosmological Context, \apjl706pp. L192-L196-L192-L196
    Lux H, Read JI, Lake G (2010) Determining Orbits for the Milky Way?s Dwarfs,American Institute of Physics Conference Series1240pp. 415-416 - 415-416
    Read JI, Wilkinson MI, Evans NW, Gilmore G, Kleyna JT (2006) The importance of tides for the Local Group dwarf spheroidals, \mnras367pp. 387-399-387-399
    Gatto A, Fraternali F, Read JI, Marinacci F, Lux H, Walch S (2013) Unveiling the corona of the Milky Way via ram-pressure stripping of dwarf satellites, MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY433(4)pp. 2749-2763 OXFORD UNIV PRESS
    Read JI, Wilkinson MI, Evans NW, Gilmore G, Kleyna JT (2006) The tidal stripping of satellites, \mnras366pp. 429-437-429-437
    Read JI, Mayer L, Brooks AM, Governato F, Lake G (2009) A dark matter disc in three cosmological simulations of Milky Way mass galaxies, Monthly Notices of the Royal Astronomical Society397(1)pp. 44-51
    Making robust predictions for the phase-space distribution of dark matter at the solar neighbourhood is vital for dark matter direct-detection experiments. To date, almost all such predictions have been based on simulations that model the dark matter alone. Here, we use three cosmological hydrodynamic simulations of bright, disc-dominated galaxies to include the effects of baryonic matter self-consistently for the first time. We find that the addition of baryonic physics drastically alters the dark matter profile in the vicinity of the solar neighbourhood. A stellar/gas disc, already in place at high redshift, causes merging satellites to be dragged preferentially towards the disc plane where they are torn apart by tides. This results in an accreted dark matter disc that contributes <0.25-1.5 times the non-rotating halo density at the solar position. The dark disc, unlike dark matter streams, is an equilibrium structure that must exist in disc galaxies that form in a hierarchical cosmology. Its low rotation lag with respect to the Earth significantly boosts Weakly Interacting Massive Particle (WIMP) capture in the Earth and Sun, boosts the annual modulation signal and leads to distinct variations in the flux as a function of recoil energy that allow the WIMP mass to be determined. © 2009 RAS.
    Agertz O, Moore B, Stadel J, Potter D, Miniati F, Read J, Mayer L, Gawryszczak A, Kravtsov A, Nordlund Å, Pearce F, Quilis V, Rudd D, Springel V, Stone J, Tasker E, Teyssier R, Wadsley J, Walder R (2007) Fundamental differences between SPH and grid methods, \mnras380pp. 963-978-963-978
    Garbari S, Read JI, Lake G (2012) Limits on the local dark matter density,European Physical Journal Web of Conferences19pp. 1008-1008
    Adén D, Wilkinson MI, Read JI, Feltzing S, Koch A, Gilmore GF, Grebel EK, Lundström I (2009) A new low mass for the Hercules dSph: The end of a common mass scale for the dwarfs?, Astrophysical Journal706(1 PART 2)
    We present a new mass estimate for the Hercules dwarf spheroidal (dSph) galaxy, based on the revised velocity dispersion obtained by Adén et al. The removal of a significant foreground contamination using newly acquired Strömgren photometry has resulted in a reduced velocity dispersion. Using this new velocity dispersion of 3.72 0.91 km s-1, we find a mass of M300 = 1.9+1.1-0.8 × 106 M™ within the central 300 pc, which is also the half-light radius, and a mass of M433 = 3.7+2.2-1.6 × 106 M™ within the reach of our data to 433 pc, significantly lower than previous estimates. We derive an overall mass-to-light ratio of M433/L = 103+83-48[M™/L™]. Our mass estimate calls into question recent claims of a common mass scale for dSph galaxies. Additionally, we find tentative evidence for a velocity gradient in our kinematic data of 16 3 km s-1 kpc-1, and evidence of an asymmetric extension in the light distribution at <0.5 kpc. We explore the possibility that these features are due to tidal interactions with the Milky Way. We show that there is a self-consistent model in which Hercules has an assumed tidal radius of rt = 485 pc, an orbital pericenter of r p = 18.5 5 kpc, and a mass within rt of Mtid,rt = 5.2+2.7-2.7 × 106 M™. Proper motions are required to test this model. Although we cannot exclude models in which Hercules contains no dark matter, we argue that Hercules is more likely to be a dark-matter-dominated system that is currently experiencing some tidal disturbance of its outer parts. © 2009 The American Astronomical Society. All rights reserved.
    Read JI, Gilmore G (2005) Mass loss from dwarf spheroidal galaxies: the origins of shallow dark matter cores and exponential surface brightness profiles, \mnras356pp. 107-124-107-124
    Knebe A, Knollmann SR, Muldrew SI, Pearce FR, Aragon-Calvo MA, Ascasibar Y, Behroozi PS, Ceverino D, Colombi S, Diemand J, Dolag K, Falck BL, Fasel P, Gardner J, Gottlöber S, Hsu CH, Iannuzzi F, Klypin A, Luki? Z, Maciejewski M, Mcbride C, Neyrinck MC, Planelles S, Potter D, Quilis V, Rasera Y, Read JI, Ricker PM, Roy F, Springel V, Stadel J, Stinson G, Sutter PM, Turchaninov V, Tweed D, Yepes G, Zemp M (2011) Haloes gone MAD: The Halo-Finder Comparison Project, Monthly Notices of the Royal Astronomical Society415(3)pp. 2293-2318
    We present a detailed comparison of fundamental dark matter halo properties retrieved by a substantial number of different halo finders. These codes span a wide range of techniques including friends-of-friends, spherical-overdensity and phase-space-based algorithms. We further introduce a robust (and publicly available) suite of test scenarios that allow halo finder developers to compare the performance of their codes against those presented here. This set includes mock haloes containing various levels and distributions of substructure at a range of resolutions as well as a cosmological simulation of the large-scale structure of the universe. All the halo-finding codes tested could successfully recover the spatial location of our mock haloes. They further returned lists of particles (potentially) belonging to the object that led to coinciding values for the maximum of the circular velocity profile and the radius where it is reached. All the finders based in configuration space struggled to recover substructure that was located close to the centre of the host halo, and the radial dependence of the mass recovered varies from finder to finder. Those finders based in phase space could resolve central substructure although they found difficulties in accurately recovering its properties. Through a resolution study we found that most of the finders could not reliably recover substructure containing fewer than 30-40 particles. However, also here the phase-space finders excelled by resolving substructure down to 10-20 particles. By comparing the halo finders using a high-resolution cosmological volume, we found that they agree remarkably well on fundamental properties of astrophysical significance (e.g. mass, position, velocity and peak of the rotation curve). We further suggest to utilize the peak of the rotation curve, vmax, as a proxy for mass, given the arbitrariness in defining a proper halo edge. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.
    Bacon DJ, Amara A, Read JI (2010) Measuring dark matter substructure with galaxy-galaxy flexion statistics, Monthly Notices of the Royal Astronomical Society409(1)pp. 389-395
    It is of great interest to measure the properties of substructures in dark matter haloes at galactic and cluster scales. Here we suggest a method to constrain substructure properties using the variance of weak gravitational flexion in a galaxy-galaxy lensing context; this is a statistical method, requiring many foreground-background pairs of galaxies. We show the effectiveness of flexion variance in measuring substructures in N-body simulations of dark matter haloes, and present the expected galaxy-galaxy lensing signals. We show the insensitivity of the method to the overall galaxy halo mass, and predict the method's signal-to-noise ratio for a space-based all-sky survey, showing that the presence of substructure down to 10 9 M haloes can be reliably detected. © 2010 The Authors. Journal compilation © 2010 RAS.
    Amendola L, Appleby S, Bacon D, Baker T, Baldi M, Bartolo N, Blanchard A, Bonvin C, Borgani S, Branchini E, Burrage C, Camera S, Carbone C, Casarini L, Cropper M, deRham C, di Porto C, Ealet A, Ferreira PG, Finelli F, Garcia-Bellido J, Giannantonio T, Guzzo L, Heavens A, Heisenberg L, Heymans C, Hoekstra H, Hollenstein L, Holmes R, Horst O, Jahnke K, Kitching TD, Koivisto T, Kunz M, La Vacca G, March M, Majerotto E, Markovic K, Marsh D, Marulli F, Massey R, Mellier Y, Mota DF, Nunes N, Percival W, Pettorino V, Porciani C, Quercellini C, Read J, Rinaldi M, Sapone D, Scaramella R, Skordis C, Simpson F, Taylor A, Thomas S, Trotta R, Verde L, Vernizzi F, Vollmer A, Wang Y, Weller J, Zlosnik T (2012) Cosmology and fundamental physics with the Euclid satellite, ArXiv e-prints
    Bruch T, Read J, Baudis L, Lake G (2009) Detecting the Milky Way?s Dark Disk, \apj696pp. 920-923-920-923
    Bruch T, Read J, Baudis L, Lake G (2008) Signatures of the Milky Way?s dark disk in current and future experiments, Identification of Dark Matter 2008
    Wilkinson MI, Kleyna JT, Wyn Evans N, Gilmore GF, Read JI, Koch A, Grebel EK, Irwin MJ (2006) The internal kinematics of dwarf spheroidal galaxies, EAS Publications Series20pp. 105-112-105-112
    Read JI, Gilmore G (2003) Can supermassive black holes alter cold dark matter cusps through accretion?, \mnras339pp. 949-956-949-956
    Read JI, Iorio G, Agertz O, Fraternali F (2016) Understanding the shape and diversity of dwarf galaxy rotation curves in LCDM,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<40deg), it is challenging to properly correct for their inclination i. This leads to a very shallow rotation curve, with a systematic underestimate of M200 of over a dex. All three problems can be easily avoided, however. Using a new coreNFW profile that accounts for cusp-core transformations, we show that we are able to successfully recover the rotation curve shape; dark matter halo mass M200; and concentration parameter c within our quoted uncertainties, provided that the galaxy is close to equilibrium. We fit our coreNFW model to four dwarf irregulars chosen to span a range of rotation curve shapes. We obtain an excellent fit for NGC 6822 and WLM. However, IC 1613 and DDO 101 give a poor fit. For IC 1613, this owes to disequilibria and its uncertain inclination; for DDO 101, the problem is its uncertain distance. Assuming i_IC1613 ~ 15deg and D_DDO101 ~ 12Mpc, we are able to fit both galaxies very well. We conclude that, once we avoid disequilibrium galaxies, or those with poorly measured inclination and/or distance, LCDM gives a remarkable match to dwarf galaxy rotation curves.
    Read JI, Lake G, Agertz O, Debattista VP (2008) Thin, thick and dark discs in ›cDM, Monthly Notices of the Royal Astronomical Society389(3)pp. 1041-1057
    In a › cold dark matter (›CDM) cosmology, the Milky Way accretes satellites into the stellar disc. We use cosmological simulations to assess the frequency of near disc plane and higher inclination accretion events, and collisionless simulations of satellite mergers to quantify the final state of the accreted material and the effect on the thin disc. On average, a Milky Way-sized galaxy has three subhaloes with v max > 80 km s -1; seven with v max > 60 km s -1 and 15 with v max > 40 km s -1 merge at redshift z s 1. Assuming isotropic accretion, a third of these merge at an impact angle ¸ < 20°and are dragged into the disc plane by dynamical friction. Their accreted stars and dark matter settle into a thick disc. The stellar thick disc qualitatively reproduces the observed thick disc at the solar neighbourhood, but is less massive by a factor <2 - 10. The dark matter disc contributes Á DDISC = 0.25 - 1Á HALO at the solar position. Although not likely to be dynamically interesting, the dark disc has important implications for the direct detection of dark matter because of its low velocity with respect to the Earth. Higher inclination encounters ¸ > 20°are twice as likely as low-inclination ones. These lead to structures that closely resemble the recently discovered inner and outer stellar haloes. They also do more damage to the Milky Way stellar disc creating a more pronounced flare, and warp; both long-lived and consistent with current observations. The most massive mergers (v max s 80 km s -1) heat the thin disc enough to produce a thick disc. These heated thin-disc stars are essential for obtaining a thick disc as massive as that seen in the Milky Way; they likely comprise some <50-90 per cent of the thick disc stars. The Milky Way thin disc must reform from fresh gas after z = 1. Only one in four of our sample Milky Way haloes experiences mergers massive and late enough to fully destroy the thin disc. We conclude that thick, thin and dark discs occur naturally within a ›CDM cosmology. © 2008 RAS.
    Goerdt T, Moore B, Read JI, Stadel J (2010) Core creation in galaxies and halos via sinking massive objects, Astrophysical Journal725(2)pp. 1707-1716
    We perform a detailed investigation into the disruption of central cusps via the transfer of energy from sinking massive objects. Constant density inner regions form at the radius where the enclosed mass approximately matches the mass of the infalling body. We explore parameter space using numerical simulations and give an empirical relation for the size of the resulting core within structures that have different initial cusp slopes. We find that infalling bodies always stall at the edge of these newly formed cores, experiencing no dynamical friction over many dynamical times. As applications, we consider the resulting decrease in the dark matter annihilation flux due to centrally destroyed cusps, and we present a new theory for the formation of close binary nuclei-the "stalled binary" model.We focus on one particularly interesting binary nucleus system, the dwarf spheroidal galaxy VCC 128 which is darkmatter dominated at all radii.We showthat its nucleiwould rapidly coalesce within a fewmillion years if it has a central dark matter cusp slope steeper than r-1. However, if its initial dark matter cusp is slightly shallower than a logslope of-0.75 at<0.1% of the virial radius, then the sinking nuclei naturally create a core equal to their observed separation and stall. This is close to the logslope measured in a recent billion particle cold darkmatter halo simulation. © 2010. The American Astronomical Society. All rights reserved.
    Debattista VP, Moore B, Quinn T, Kazantzidis S, Maas R, Mayer L, Read J, Stadel J (2008) The Causes of Halo Shape Changes Induced by Cooling Baryons: Disks versus Substructures, \apj681pp. 1076-1088-1076-1088
    Read JI, Moore B (2005) Tidal streams in a MOND potential: constraints from Sagittarius, \mnras361pp. 971-976-971-976
    Rojas-Nino A, Read JI, Aguilar A, Delorme M (2016) An efficient positive potential-density pair expansion for modelling galaxies,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.
    Read JI, Trentham N (2005) The baryonic mass function of galaxies, Royal Society of London Philosophical Transactions Series A363pp. 2693-2693
    Read JI, Goerdt T, Moore B, Pontzen AP, Stadel J, Lake G (2006) Dynamical friction in constant density cores: a failure of the Chandrasekhar formula, \mnras373pp. 1451-1460-1451-1460
    Read J, Debattista V, Agertz O, Mayer L, Brooks AM, Governato F, Lake G (2008) A dark matter disc in the Milky Way, Identification of Dark Matter 2008
    Saha P, Read JI (2009) The cluster lens ACO 1703: Redshift contrast and the inner profile, Astrophysical Journal690(1)pp. 154-162
    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 Á r -1 out to 200 kpc. But if the quint is omitted, both shallower and steeper models (e.g., Á 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. © 2009. The American Astronomical Society. All rights reserved.
    Bruch T, Peter AHG, Read J, Baudis L, Lake G (2009) Dark matter disc enhanced neutrino fluxes from the Sun and Earth, Physics Letters B674pp. 250-256-250-256
    Wilkinson MI, Kleyna JT, Evans NW, Gilmore GF, Grebel EK, Koch A, Read J, Young R (2005) Substructure in dwarf spheroidals - a star cluster connection?, IAU Colloq. 198: Near-fields cosmology with dwarf elliptical galaxiespp. 240-243-240-243
    Dehnen W, Read JI (2011) N-body simulations of gravitational dynamics, European Physical Journal Plus126(5)pp. 1-28
    We describe the astrophysical and numerical basis of N-body simulations, both of collisional stellar systems (dense star clusters and galactic centres) and collisionless stellar dynamics (galaxies and large-scale structure). We explain and discuss the state-of-the-art algorithms used for these quite different regimes, attempt to give a fair critique, and point out possible directions of future improvement and development. We briefly touch upon the history of N-body simulations and their most important results. © Società Italiana di Fisica / Springer-Verlag 2011.
    Evans NW, Wilkinson MI, Kleyna JT, Read JI, Gilmore G (2005) Kinematics and M/L ratios of dwarf spheroidals, IAU Colloq. 198: Near-fields cosmology with dwarf elliptical galaxiespp. 60-67-60-67
    Charbonnier A, Combet C, Daniel M, Funk S, Hinton JA, Maurin D, Power C, Read JI, Sarkar S, Walker MG, Wilkinson MI (2011) Dark matter profiles and annihilation in dwarf spheroidal galaxies: Prospectives for present and future³-ray observatories - I. The classical dwarf spheroidal galaxies, Monthly Notices of the Royal Astronomical Society418(3)pp. 1526-1556
    Due to their large dynamical mass-to-light ratios, dwarf spheroidal galaxies (dSphs) are promising targets for the indirect detection of dark matter (DM) in ³-rays. We examine their detectability by present and future ³-ray observatories. The key innovative features of our analysis are as follows: (i) we take into account the angular size of the dSphs; while nearby objects have higher ³-ray flux, their larger angular extent can make them less attractive targets for background-dominated instruments; (ii) we derive DM profiles and the astrophysical J-factor (which parametrizes the expected ³-ray flux, independently of the choice of DM particle model) for the classical dSphs directly from photometric and kinematic data. We assume very little about the DM profile, modelling this as a smooth split-power-law distribution, with and without subclumps; (iii) we use a Markov chain Monte Carlo technique to marginalize over unknown parameters and determine the sensitivity of our derived J-factors to both model and measurement uncertainties; and (iv) we use simulated DM profiles to demonstrate that our J-factor determinations recover the correct solution within our quoted uncertainties. Our key findings are as follows: (i) subclumps in the dSphs do not usefully boost the signal; (ii) the sensitivity of atmospheric Cherenkov telescopes to dSphs within ~20kpc with cored haloes can be up to ~50 times worse than when estimated assuming them to be point-like. Even for the satellite-borne Fermi-Large Area Telescope (Fermi-LAT), the sensitivity is significantly degraded on the relevant angular scales for long exposures; hence, it is vital to consider the angular extent of the dSphs when selecting targets; (iii) no DM profile has been ruled out by current data, but using a prior on the inner DM cusp slope 0 d³priord 1 provides J-factor estimates accurate to a factor of a few if an appropriate angular scale is chosen; (iv) the J-factor is best constrained at a critical integration angle ±c= 2rh/d (where rh is the half-light radius and d is the distance from the dwarf) and we estimate the corresponding sensitivity of ³-ray observatories; (v) the 'classical' dSphs can be grouped into three categories: well constrained and promising (Ursa Minor, Sculptor and Draco), well constrained but less promising (Carina, Fornax and Leo I), and poorly constrained (Sextans and Leo II); and (vi) observations of classical dSphs with the Fermi-LAT integrated over the mission lifetime are more pro
    Read JI, Agertz O, Collins MLM (2016) Dark matter cores all the way down,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.
    Gualandris A, Read JI, Dehnen W, Bortolas E (2017) Collisionless loss-cone refilling: there is no final parsec problem,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?.
    Stellar feedback refers to the injection of energy, momentum and mass into the interstellar medium (ISM) by massive stars. This feedback owes to a combination of ionising radiation, radiation pressure, stellar winds and supernovae and is likely responsible both for the inefficiency of star formation in galaxies, and the observed super-sonic turbulence of the ISM. In this thesis, I study how stellar feedback shapes the ISM thereby regulating galaxy evolution. In particular, I focus on three key questions: (i) How does stellar feedback shape the gas density distribution of the ISM? (ii) How does feedback change or influence the distribution of the kinetic energy in the ISM? and (iii) What role does feedback play in determining the star formation efficiency of giant molecular clouds (GMCs)? To answer these questions, I run high resolution (dx~4.6 pc) numerical simulations of three isolated galaxies, both with and without stellar feedback. I compare these simulations to observations of six galaxies from The HI Nearby Galaxy Survey (THINGS) using power spectra, and I use clump finding techniques to identify GMCs in my simulations and calculate their properties. I find that the kinetic energy power spectra in stellar feedback- regulated galaxies, regardless of the galaxy's mass and size, show scalings in excellent agreement with supersonic turbulence on scales below the thickness of the HI layer. I show that feedback influences the gas density field, and drives gas turbulence, up to large (kiloparsec) scales. This is in stark contrast to the density fields generated by large-scale gravity-only driven turbulence (i.e. without stellar feedback). Simulations with stellar feedback are able to reproduce the internal properties of GMCs such as: mass, size and velocity dispersion. Finally, I demonstrate that my simulations naturally reproduce the observed scatter (3.5-4 dex) in the star formation efficiency per free-fall time of GMCs, despite only employing a simple Schmidt star formation law. I conclude that the neutral gas content of galaxies carries signatures of stellar feedback on all scales and that stellar feedback is, therefore, key to regulating the evolution of galaxies over cosmic time.
    Read JI, Iorio G, Agertz O, Fraternali F (2017) The stellar mass?halo mass relation of isolated field dwarfs: a critical test of ›CDM at the edge of galaxy formation,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 << M?/M << 108 . By accounting for cusp-core transformations due to stellar feedback, we find a monotonic relation with little scatter. Such monotonicity implies that abundance matching should yield a similar M? ? M200 if the cosmological model is correct. Using the ?field galaxy? stellar mass function from the Sloan Digital Sky Survey (SDSS) and the halo mass function from the › Cold Dark Matter Bolshoi simulation, we find remarkable agreement between the two. This holds down to M200 < 5 × 109 M , and to M200 < 5 × 108 M if we assume a power law extrapolation of the SDSS stellar mass function below M? < 107 M . However, if instead of SDSS we use the stellar mass function of nearby galaxy groups, then the agreement is poor. This occurs because the group stellar mass function is shallower than that of the field below M? < 109 M , recovering the familiar ?missing satellites? and ?too big to fail? problems. Our result demonstrates that both problems are confined to group environments and must, therefore, owe to ?galaxy formation physics? rather than exotic cosmology. Finally, we repeat our analysis for a › Warm Dark Matter cosmology, finding that it fails at 68% confidence for a thermal relic mass of mWDM < 1.25 keV, and mWDM < 2 keV if we use the power law extrapolation of SDSS. We conclude by making a number of predictions for future surveys based on these results.
    Grisdale Kearn, Agertz Oscar, Romeo AB, Renaud Florent, Read Justin (2016) The impact of stellar feedback on the density and velocity structure of the interstellar medium,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.
    Silverwood H, Sivertsson S, Steger P, Read JI, Bertone G (2016) A non-parametric method for measuring the local dark matter density,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.
    Petts J, Read JI, Gualandris A (2016) A semi-analytic dynamical friction model for cored galaxies,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.
    Iorio G, Fraternali F, Nipoti C, Di Teodoro E, Read J, Battaglia G (2016) LITTLE THINGS in 3D: robust determination of the circular velocity of dwarf irregular galaxies.,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 (Ãv) 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 Ãv) that we make publicly available. These can be used to perform dynamical studies and improve our understanding of these low-mass galaxies.
    Carrera R, Conn B, Noel N, Read J, Sánchez Á (2017) The Magellanic Inter-Cloud Project (MAGIC) III: First spectroscopic evidence of a dwarf stripping a dwarf,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.
    Read Justin, Steger P. (2017) How to break the density-anisotropy degeneracy in spherical stellar systems,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 Á(r) and velocity anisotropy ²(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, Á(R1/2), but is not able to measure Á(r) or ²(r), even with 10,000 tracer stars. We then test three popular methods for breaking this Á ? ² 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 Á(r) in-between their respective R1/2. However, even with a total of <7, 000 tracers, we are not able to well-constrain ²(r) for either population. By contrast, using 1000 tracers with higher order VSPs we are able to measure Á(r) over the range 0.5 < r/R1/2 < 2 and broadly constrain ²(r). Including proper motion data for all stars gives an even better performance, with Á and ² 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.
    Contenta Filippo, Balbinot Eduardo, Petts James, Read Justin, Gieles Mark, Collins Michelle, Peñarrubia Jorge, Delorme Maxime, Gualandris Alessia (2018) Probing dark matter with star clusters: a dark matter core in the ultra-faint dwarf Eridanus II,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.
    Ciuc? Ioana, Kawata Daisuke, Ando Shin?ichiro, Calore Francesca, Read Justin I, Mateu Cecilia (2018) A Gaia DR2 search for dwarf galaxies towards Fermi-LAT sources: implications for annihilating dark matter,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.
    Read J I, Walker M G, Steger P (2018) The case for a cold dark matter cusp in Draco,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, Á _DM(150 pc). We show that, combined with constraints on the density profile at larger radii, this is sufficient to distinguish a › Cold Dark Matter (›CDM) cusp - that has Á _DM(150 pc) s 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 Á _DM(150 pc) = 2.4_{-0.6}^{+0.5} × 10^8 M_ kpc^{-3}, consistent with a ›CDM cusp. Using a velocity-independent SIDM model, calibrated on ›SIDM cosmological simulations, we show that Draco's high central density gives an upper bound on the SIDM cross-section of Ã/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.
    Sivertsson S, Silverwood H, Read J I, Bertone G, Steger P (2018) The localdark matter density from SDSS-SEGUE G-dwarfs,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 ±-young population from Büdenbender et al. yields the most reliable result of Á_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 ±-young and ±-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.
    Rey-Raposo Ramon, Read Justin I (2018) The alignment is in their stars: on the spin-alignment of stars in star clusters,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.
    Mateu Cecilia, Read Justin I, Kawata Daisuke (2017) Fourteen candidate RR Lyrae star streams in the inner Galaxy,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 >4Ã confidence and another 12 at >3.5Ã 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.
    Bermejo-Climent José R, Battaglia Giuseppina, Gallart Carme, Di Cintio Arianna, Brook Chris B, Cicuéndez Luis, Monelli Matteo, Leaman Ryan, Mayer Lucio, Peñarrubia Jorge, Read Justin I (2018) On the early evolution of Local Group dwarf galaxy types: star formation and supernova feedback,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.
    Forbes Duncan A, Read Justin I, Gieles Mark, Collins Michelle L M (2018) Extending the globular cluster system?halo mass relation to the lowest galaxy masses,Monthly Notices of the Royal Astronomical Society481(4)pp. 5592-5605
    High-mass galaxies, with halo masses M200 e 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 < 1014M ™ ™ down to below M200 <109M ™ ™ , 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.
    Read J I, Walker M G, Steger P (2019) Dark matter heats up in dwarf galaxies,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, ÁDM(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 ÁDM(150pc)>108M™ kpc?3, consistent with cold dark matter cusps, while those with more extended star formation favour ÁDM(150pc)<108M™ kpc?3, consistent with shallower dark matter cores. Using abundance matching to infer pre-infall halo masses, M200, we show that this dichotomy is in excellent agreement with models in which dark matter is heated up by bursty star formation. In particular, we find that ÁDM(150pc) steadily decreases with increasing stellar mass-to-halo mass ratio, M*/M200. Our results suggest that, to leading order, dark matter is a cold, collisionless, fluid that can be kinematically ?heated up? and moved around.
    Gregory Alexandra L, Collins Michelle L M, Read Justin, Irwin Michael J, Ibata Rodrigo A, Martin Nicolas F, McConnachie Alan W, Weisz Daniel R (2019) Kinematics of the Tucana Dwarf Galaxy: an unusually dense dwarf in the Local Group,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 Ãv,Tuc=14.4+2.8?2.3Ãv,Tuc=14.4?2.3+2.8 km s?1. We also detect a rotation gradient of dvrdÇ=7.6+4.2?4.3dvrdÇ=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 Ãv,Tuc=13.3+2.7?2.3Ãv,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 › 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.
    Read J. I., Erkal D. (2019) Abundance matching with the mean star formation rate: there is no missing satellites problem in the Milky Way above M200<109M™,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, èSFRé, 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 èSFRé?M200 relation in ›CDM. 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 èSFRé?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.
    Orkney M D A, Read J I, Petts J A, Gieles M (2019) Globular clusters as probes of dark matter cusp-core transformations,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 à 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* C 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* C 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.
    Wan Zhen, Oliver William H, Lewis Geraint F, Read Justin, Collins Michelle (2019) On the origin of the asymmetric dwarf galaxy distribution around andromeda,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.
    Chakrabarti Sukanya, Chang Philip, Price-Whelan Adrian M., Read Justin, Blitz Leo, Hernquist Lars (2019) Antlia 2?s Role in Driving the Ripples in the Outer Gas Disk of the Galaxy,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 (¼ ± cos´, ¼ ´ ) = (?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.
    Rey Martin P., Pontzen Andrew, Agertz Oscar, Orkney Matthew, Read Justin, Saintonge Amélie, Pedersen Christian (2019) EDGE: The Origin of Scatter in Ultra-faint Dwarf Stellar Masses and Surface Brightnesses,Astrophysical Journal Letters886(1) 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 (ý1 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 (MV = -5.7) dwarf galaxies within the reach of next-generation low surface brightness surveys.
    Agertz Oscar, Pontzen Andrew, Read Justin, Rey Martin P, Orkney Matthew, Rosdahl Joakim, Teyssier Romain, Verbeke Robbert, Kretschmer Michael, Nickerson Sarah (2019) EDGE: the mass?metallicity relation as a critical test of galaxy formation physics,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.
    One of the theories of galactic evolution is that large structures grow, hierarchically, from the merger and accretion of smaller ones. Evidence for this lies in the spectacular stellar streams observed around the Milky Way, Andromeda and other galaxies within and beyond the Local Group. In this thesis I study two stellar structures in the outer halo of Andromeda: the North West Stream (NW Stream) and an intriguing filamentary feature, the Eastern Extent located proximate the Giant Stellar Stream. The NW Stream comprises two separate segments thought to be parts of a single structure entwined around Andromeda. The upper segment of the NW Stream is co-located on-sky with the dwarf Spheroidal galaxy Andromeda XXVII. Using data from the Pan-Andromeda Astronomical Survey, I compare the kinematic and spectroscopic properties of these two features and find it plausible that Andromeda XXVII is being tidally disrupted by Andromeda and could be the progenitor of the upper segment of the NW Stream. Comparing velocity gradients and model orbits for both segments of the NW Stream, I find evidence that they are both infalling towards Andromeda indicating that it is unlikely that the NW Stream is a single structure. I also derive predictions for the proper motions for both stream seg- ments, which I find to be consistent with similar predictions for Andromeda and Andromeda XXVII. With respect to the Eastern Extent, I find it has kinematic and photometric properties consistent with other nearby features, i.e. Stream B and one of the substructures in Stream C, Stream Cr, plausibly linking these features. When I compare my results for the Eastern Extent to the properties of the Giant Stellar Stream I find that the Eastern Extent could plausibly comprise stars stripped from the progenitor of the Giant Stellar Stream. I conclude that further modelling is required to determine how the Giant Stellar Stream, the Eastern Extent and Streams B and Cr were formed.
    Nasim Imran, Gualandris Alessia, Read Justin, Dehnen Walter, Delorme Maxime, Antonini Fabio (2020) Defeating stochasticity: coalescence timescales of massive black holes in galaxy mergers,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 ý 5ý105, 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 ý 107 within the stellar half-light radius suffices to reduce the scatter in the merger timescale to ý 10%. Our results suggest that at least some of the uncertainty in low-frequency GW rates owes to insufficient numerical resolution.
    De Leo Michele, Carrera Ricardo, Noel Noelia E.D, Read Justin I., Erkal Denis, Gallart Carme (2020) Revealing the tidal scars of the Small Magellanic Cloud,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.

    Additional publications