The imprint of galaxy formation and the nature of dark matter on galactic scales

Abstract

The standard model of cosmology assumes most matter is made up of cold dark matter (CDM), which provides an excellent agreement between observations and predictions concerning the large-scale structure of the Universe. On smaller scales, discrepancies in its sub-galactic distribution exist, motivating alternative dark matter (DM) models. These scales are nonetheless poorly resolved in cosmological simulations and susceptible to the effects of galaxy formation. This thesis explores both issues in the context of Milky Way-mass haloes (MW) and their satellite population. We first investigate the role of internal processes of galaxies hosted in DM halos in shaping their central DM density, finding that stellar bars and gas blowouts driven by Active Galactic Nuclei counteract gravitational contraction. As both of these components reflect the evolutionary history of their host, this underscores the importance of accounting for assembly histories when modelling contracted haloes. The second part of this thesis compares the properties of the satellite population of MWs across cold, warm (WDM) and self-interacting (SIDM) dark matter models, finding that the assumed nature of the DM affects their abundance, characteristic mass function, and spatial distribution. This reflects a suppression in the number of galaxies forming (WDM) and more tidal stripping (SIDM) compared to CDM. As the stellar halo is built from the stripped remnants, its spatial distribution and phase-space structure are also affected. However, certain galaxy formation models result in similar structural differences. Given the uncertainty in how to model star formation, this hinders the use of the brightest satellites as powerful constraints on the nature of DM. Conversely, haloes hosting ultra-faint galaxies are less affected by these degeneracies, making them better constraints on the nature of DM. These are, nonetheless, poorly resolved in simulations, complicating comparisons to real data. We conclude this thesis by discussing preparations targeted towards running the highest-resolution simulation of a MW halo in CDM and WDM to date, which will provide unbiased predictions concerning its satellite population. Together with upcoming surveys, which will provide the deepest characterisation of the real MW satellite population as of yet, this will enable the use of ultra-faints to further constrain the nature of the DM.