The merger history of the Milky Way and its dwarf galaxies

Abstract

This thesis uses simulations to study the assembly history of the Milky Way (MW), make predictions for the upcoming James Webb Space Telescope (JWST) surveys and to identify the best regions in the universe to study the nature of dark matter using dwarf galaxies.

Chapter 2 presents the properties of simulated MW-mass halos constrained to have accretion histories similar to that of the MW, specifically the recent accretion of a Large Magellanic Cloud (LMC)-like galaxy and a merger analogous to the Gaia-Enceladus Sausage (GES), with a quiescent period between the GES merger and the infall of the LMC (the `LMC and GES' category). We find that ~16 per cent of MW-mass halos have an LMC; ~5 per cent have a GES event and no further merger with an equally massive object since z=1; and only 0.65 per cent belong to the LMC and GES category. The progenitors of the MWs in this last category are much less massive than average at early times but eventually catch up with the mean. The LMC and GES category of galaxies naturally end up in the `blue cloud' in the colour–magnitude diagram at z=0, tend to have a disc morphology and have a larger than average number of satellite galaxies.

Chapter 3 presents predictions, derived from the EAGLE LCDM cosmological hydrodynamical simulations, for the abundance and properties of galaxies expected to be detected at high redshift by JWST. We consider the galaxy population as a whole and focus on the sub-population of progenitors of MW analogues, defined to be galaxies with accretion histories similar to the MW's, that is, galaxies that underwent a merger resembling the GES event and that contain an analogue of the LMC satellite today. We derive the luminosity function of all EAGLE galaxies in JWST/NIRCam passbands, in the redshift range z = 2 - 8, taking into account dust obscuration and different exposure times. For an exposure time of T = 10^5 s, average MW progenitors are observable as far back as z~6 in most bands, and this changes to z~5 and z~4 for the GES and LMC progenitors, respectively. The progenitors of GES and LMC analogues are, on average, ~2 and ~1 mag fainter than the MW progenitors at most redshifts. They lie, on average, within ~60 and 30 arcsec, respectively, of their future MW host at all times, and thus will appear within the field of view of JWST/NIRCam. We conclude that galaxies resembling the main progenitor of the MW and its major accreted components should be observable with JWST beyond redshift 2, providing a new and unique window in studying the formation history of our own galaxy.

Chapter studies the effect of environment on the merger rates of dark matter halos, with an emphasis on the mass regime relevant for dwarf galaxies (M_halo=10^9-10^11 Msun) in both a cold and warm dark matter cosmology. A halo's environment has been explored using two different methods, 1) probing the cosmic web geometry using the NEXUS algorithm and 2) probing the local environment using halo counts. We find that the nature of dark matter has a much bigger impact on halo mergers than any environmental differences. Low mass halos (M_halo=10^9-10^9.5 Msun) have roughly 9 (5) times more minor (major) mergers in CDM than in WDM. We also find that there are differences between the assembly histories of 10^11-10^12 Msun mass halos in CDM and WDM. In CDM, halos would have experienced roughly three times more major mergers in their lifetime. The environmental impact on halo mergers affects when a halo experiences its mergers, but not how many it has in its lifetime. Furthermore, the large-scale structure has a bigger impact on early mergers, and the local environment has a bigger impact on recent mergers. We find that low mass halos (10^9-10^9.5 Msun) are up to four times more likely to have experienced a recent major merger in a low local environment density in CDM than in WDM. Therefore, a low density local scale environment would be the ideal regime for observing the differences between CDM and WDM using dwarf galaxies.