Justin Read

Prof Justin Read


Head of the Astrophysics Research Group, Professor
+44 (0)1483 683479

Academic and research departments

Astrophysics Research Group.

Biography

Biography

Prof. Justin 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, UK. He was recently awarded the MERAC Prize by the European Astronomical Society for his high impact research in computational astrophysics and cosmology.

Publications

A full list of publications (freely available for download) is available on arXiv.org.

Links

Teaching

Prof. Read is module coordinator for the Physics with Astronomy degree programme. He also teaches:

  • PHY2071: Introduction to Astronomy
  • PHYM059 : Astrophysical Dynamics
  • Year 1 Small Group Tutorials

University roles and responsibilities

  • Chair IT Governance Group
  • Chair High Performance Computing Stakeholder Group
  • Head of Astrophysics
  • Module coordinator for the Physics with Astronomy degree programme

My publications

Publications

Bharmal NA, Buscher DF, Haniff CA, Read JI (2003) A novel wavefront sensor for interferometry, Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series 4838 pp. 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 Series 1240 pp. 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, Monthly Notices of the Royal Astronomical Society 422 (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
Read JI, Lake G, Agertz P, Debattista V (2008) A dark disc in the Milky Way, Astronomische Nachrichten 329 (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
Ural U, Wilkinson MI, Read JI, Walker MG (2015) A low pre-infall mass for the Carina dwarf galaxy from disequilibrium modelling., Nat Commun 6 pp. 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
Read JI, Hayfield T (2012) SPHS: Smoothed particle hydrodynamics with a higher order dissipation switch, Monthly Notices of the Royal Astronomical Society 422 (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 Catalog 741 pp. 81526-81526
Read JI, Pontzen AP, Viel M (2006) On the formation of dwarf galaxies and stellar haloes, \mnras 371 pp. 885-897-885-897
Read JI, Saha P, Macciò AV (2007) Radial Density Profiles of Time-Delay Lensing Galaxies, \apj 667 pp. 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: Letters 424 (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, \apjl 652 pp. L5-L8-L5-L8
Read JI (2004) Dwarf spheroidal galaxies - the key to unlocking the nature of dark matter?, The Observatory 124 pp. 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?, \mnras 368 pp. 1073-1077-1073-1077
Read J (2007) Parameterized equations of state for neutron stars, APS Meeting Abstracts pp. 11005-11005
Lux H, Read JI, Lake G (2010) Determining orbits for the Milky Way's dwarfs, Monthly Notices of the Royal Astronomical Society 406 (4) pp. 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
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 galaxies pp. 235-239-235-239
Garbari S, Read JI, Lake G (2011) Limits on the local dark matter density, Monthly Notices of the Royal Astronomical Society 416 (3) pp. 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 Society 405 (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 Society 429 (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 Series 1240 pp. 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 Society 425 (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
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 Society 426 (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 (
Boley AC, Lake G, Read J, Teyssier R (2009) Globular Cluster Formation Within a Cosmological Context, \apjl 706 pp. L192-L196-L192-L196
Lux H, Read JI, Lake G (2010) Determining Orbits for the Milky Way?s Dwarfs, American Institute of Physics Conference Series 1240 pp. 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, \mnras 367 pp. 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 SOCIETY 433 (4) pp. 2749-2763 OXFORD UNIV PRESS
Read JI, Wilkinson MI, Evans NW, Gilmore G, Kleyna JT (2006) The tidal stripping of satellites, \mnras 366 pp. 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 Society 397 (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
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, \mnras 380 pp. 963-978-963-978
Garbari S, Read JI, Lake G (2012) Limits on the local dark matter density, European Physical Journal Web of Conferences 19 pp. 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 Journal 706 (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
Read JI, Gilmore G (2005) Mass loss from dwarf spheroidal galaxies: the origins of shallow dark matter cores and exponential surface brightness profiles, \mnras 356 pp. 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 Society 415 (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 Society 409 (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, \apj 696 pp. 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 Series 20 pp. 105-112-105-112
Read JI, Gilmore G (2003) Can supermassive black holes alter cold dark matter cusps through accretion?, \mnras 339 pp. 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 Society 462 (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
Read JI, Lake G, Agertz O, Debattista VP (2008) Thin, thick and dark discs in ›cDM, Monthly Notices of the Royal Astronomical Society 389 (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°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
Goerdt T, Moore B, Read JI, Stadel J (2010) Core creation in galaxies and halos via sinking massive objects, Astrophysical Journal 725 (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
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, \apj 681 pp. 1076-1088-1076-1088
Read JI, Moore B (2005) Tidal streams in a MOND potential: constraints from Sagittarius, \mnras 361 pp. 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 Society 459 (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 A 363 pp. 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, \mnras 373 pp. 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 Journal 690 (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 B 674 pp. 250-256-250-256
Noel NED, Conn BC, Carrera R, Read JI, Rix H-W, Dolphin A (2013) THE MAGELLANIC INTER-CLOUD PROJECT (MAGIC). I. EVIDENCE FOR INTERMEDIATE-AGE STELLAR POPULATIONS IN BETWEEN THE MAGELLANIC CLOUDS, ASTROPHYSICAL JOURNAL 768 (2) ARTN 109 IOP PUBLISHING LTD
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 galaxies pp. 240-243-240-243
Dehnen W, Read JI (2011) N-body simulations of gravitational dynamics, European Physical Journal Plus 126 (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 galaxies pp. 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 Society 418 (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 Society 459 pp. 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 Society 464 (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
Grisdale KM (2017) The role of stellar feedback on the structure of the ISM and star formation in galaxies.,
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 Society 467 (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
Grisdale KM, Agertz O, Romeo A, Renaud F, Read JI (2016) The impact of stellar feedback on the density and velocity structure of the interstellar medium, Monthly Notices of the Royal Astronomical Society 466 (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
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 >
Petts J, Read JI, Gualandris A (2016) A semi-analytic dynamical friction model for cored galaxies, Monthly Notices of the Royal Astronomical Society 463 (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 Society 466 (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 Society 471 (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 J, Steger P (2017) How to break the density-anisotropy degeneracy in spherical stellar systems, Monthly Notices of the Royal Astronomical Society 471 (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
Contenta F, Balbinot E, Petts J, Read J, Gieles M, Collins M, Peñarrubia J, Delorme M, Gualandris A (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.