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Effect of Baryons on the Dark Matter Distribution from Cosmological

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Effect of Baryons on the Dark Matter Distribution from Cosmological Effects of baryons on the dark matter distribution in cosmological hydrodynamical simulations Matthieu Schaller Abstract Simulations including solely dark matter performed over the last three decades have delivered an accurate and robust description of the cosmic web and dark mat- ter structures. With the advent of more precise cosmological probes, planned and ongoing, and dark matter detection experiments, this numerical modelling has to be improved to incorporate the complex non-linear and energetic processes taking place during galaxy formation. We use the “Evolution and Assembly of GaLaxies and their Environment” (EAGLE) suite of cosmological simulations to investigate the effects of baryons and astrophysical processes on the underlying dark matter distribution. Many effects are expected and we investigate (i): the modification of the profile of halos from the Navarro-Frenk-White profile shape found in collision- less simulations, including the changes in the dark matter profiles themselves, (ii) the changes of the inner density profiles of rich clusters, where observations have suggested a deviation from the standard cold dark matter paradigm, (iii) the off- set created by astrophysical process between the centre of galaxies and the centre of the dark matter halo in which they reside and, (iv) the changes in the shape of the dark matter profile due to baryons in the centre of Milky Way halos and the impact these changes have on the morphology of the annihilation signal that could be observed as an indirect proof of the existence of dark matter. In all cases we find that the baryons play a significant role and change the results found in colli- sionless simulations dramatically. This highlights the need for more simulations like EAGLE to better understand and analyse future cosmology surveys. We also conduct a thorough study of the hydrodynamics solver parameters used in these simulations, assess their impact on the simulated galaxy population and show how robust some of the EAGLE results are against such variations. Effects of baryons on the dark matter distribution in cosmological hydrodynamical simulations by Matthieu Schaller A thesis submitted to the University of Durham in accordance with the regulations for admittance to the Degree of Doctor of Philosophy. Department of Physics University of Durham June 2015 Contents List of Figuresv List of Tables viii Declarationx Acknowledgements xiii 1 Introduction1 1.1 Cosmology, Dark matter and Galaxy formation.............1 1.2 The role of simulations...........................7 1.3 Thesis outline................................ 13 2 The EAGLE suite of simulations 16 2.1 Gravity and Hydrodynamics solver................... 18 2.2 Subgrid models............................... 21 2.3 Other scientific results from EAGLE .................... 26 3 Importance of the hydrodynamics scheme 30 3.1 Introduction................................. 30 3.2 The EAGLE simulations.......................... 32 3.2.1 Subgrid models and halo identification............. 33 3.2.2 SPH implementations....................... 35 3.2.3 Thermal energy injection and time-step limiter......... 40 3.3 Galaxy population and evolution through cosmic time........ 42 i 3.3.1 The galaxy stellar mass function................. 43 3.3.2 The sizes of galaxies........................ 47 3.3.3 The star formation rate of galaxies................ 50 3.4 Large- and small-scale gas distribution.................. 56 3.4.1 Gas in large-scale structures.................... 57 3.4.2 Extragalactic gas in haloes..................... 59 3.4.3 ISM and CGM gas......................... 67 3.5 Summary & Conclusion.......................... 71 4 Baryon effects on the internal structure of ΛCDM halos 74 4.1 Introduction................................. 74 4.2 The simulations............................... 77 4.2.1 Baryonic physics.......................... 79 4.2.2 Halo definition and selection................... 82 4.2.3 Matching halos between the two simulations.......... 83 4.3 Halo masses and content.......................... 83 4.3.1 The effect of baryon physics on the total halo mass...... 84 4.3.2 The halo mass function...................... 89 4.3.3 Baryonic and stellar fractions in the EAGLE simulation.... 91 4.4 Halo profiles................................ 93 4.4.1 Resolution and convergence considerations........... 95 4.4.2 Stacked halo density and cumulative mass of relaxed halos. 99 4.4.3 Halo circular velocities....................... 102 4.4.4 An empirical universal density profile.............. 105 4.4.5 Dark matter density profile.................... 111 4.4.6 Halo concentrations........................ 113 4.4.7 Best-fit parameter values for the new density profile...... 119 4.4.8 A non-parametric estimate of the concentration........ 123 4.4.9 Limitations of the subgrid model................. 125 4.5 Conclusions................................. 127 ii 5 The effect of baryons on the inner density profiles of rich clusters 131 5.1 Introduction................................. 131 5.2 The EAGLE simulations.......................... 134 5.2.1 Baryon physics........................... 134 5.2.2 Photometry............................. 135 5.3 The mass density profile of clusters.................... 136 5.3.1 The mass density profiles of simulated halos.......... 139 5.3.2 Fitting models to the simulated halos.............. 140 5.4 The inner density profile.......................... 143 5.4.1 Total mass profiles: simulation results.............. 144 5.4.2 Total mass profiles: overview of recent observational data.. 147 5.4.3 Dark matter density profiles.................... 150 5.5 Discussion.................................. 151 5.5.1 Surface brightness profiles..................... 152 5.5.2 Velocity dispersion profiles.................... 155 5.5.3 Mass-to-light ratio......................... 158 5.6 Summary and conclusions......................... 163 6 The offsets between galaxies and their dark matter in ΛCDM 167 6.1 Introduction................................. 167 6.2 Method.................................... 170 6.2.1 The simulation suite........................ 170 6.2.2 Identification of galaxies and their locations.......... 171 6.3 Offsets between dark matter and stars.................. 172 6.3.1 3D offset between dark matter and stellar components.... 172 6.3.2 Offset along the direction of motion............... 174 6.3.3 Detailed examination of satellite galaxies in the tail of the dis- tribution............................... 176 6.3.4 Detailed examination of satellite galaxies in the cores of clusters177 6.4 Summary & Conclusion.......................... 178 iii 7 Morphology of the dark matter annihilation signal around the galactic centre of simulated Milky Way galaxies 181 7.1 Introduction................................. 181 7.2 The simulations............................... 185 7.2.1 Simulation code and subgrid models.............. 185 7.2.2 Selection of Milky-Way halos................... 186 7.3 Dark matter distribution in the centre of the halos........... 187 7.3.1 Profiles without baryon effects.................. 187 7.3.2 Profiles in the simulations with baryons............. 189 7.3.3 Origin of the central flattening.................. 193 7.3.4 Sphericity of the distribution................... 193 7.3.5 Position of the centre........................ 194 7.4 Dark matter annihilation signal...................... 195 7.4.1 J-factor for the simulated halos.................. 195 7.4.2 Extrapolation of the profiles towards the centre........ 196 7.4.3 Gamma-ray flux morphology................... 198 7.5 Summary & Conclusion.......................... 199 8 Conclusions 202 8.1 Summary of the results........................... 202 8.2 Future work................................. 205 A Uncertainties due to the subgrid models 208 A.1 Changes in the AGN model parameters................. 208 Bibliography 211 iv List of Figures 3.1 The z = 0:1 GSMF of the simulations using ANARCHY SPH and GAD- GET SPH................................... 44 3.2 The ratio between stellar mass and halo mass for the ANARCHY SPH and GADGET SPH simulations....................... 46 3.3 The relation between stellar mass and galaxy size for the simulations using ANARCHY SPH and GADGET SPH................. 48 3.4 The SSFR of galaxies at z = 0:1 in the GADGET and ANARCHY simu- lations.................................... 51 3.5 The SSFR of galaxies at z = 0:1 in the GADGET and ANARCHY simu- lations.................................... 53 3.6 The fraction of passive galaxies at z = 0:1 in the GADGET and ANAR- CHY simulations............................... 55 3.7 The distribution of gas in the temperature-density plane for both the GADGET and ANARCHY simulations................... 58 3.8 Star formation rate in the intra-group medium as a function of halo mass for the GADGET and ANARCHY simulations............ 60 3.9 Dense gas column density mass of the largest halos in the GADGET and ANARCHY simulations......................... 62 3.10 Gas fraction as a function of halo mass for the GADGET and ANAR- CHY simulations............................... 64 3.11 I-Band luminosity as a function of halo mass for the GADGET and ANARCHY simulations........................... 66 3.12 The HI mass function of both the GADGET and ANARCHY simulations 68 v 3.13 The distribution of gas pressure at which the stars were born in the GADGET and ANARCHY simulations................... 70 4.1 The ratio of the masses of the matched halos in the EAGLE and DMO simulations.................................
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