Merger histories of dark matter halos in ΛCDM and implications for the evolution of Milky Way-size galaxies

Details Merger histories of dark matter halos in ΛCDM and implications for the evolution of Milky Way-size galaxies Merger histories of dark matter halos in ΛCDM and implications for the evolution of Milky Way-size galaxies

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009phdt.......249s&link_type=abstract

PhD Thesis, University of California Irvine, 2009

Mathematics

Logic

Scientific paper

There is a concern in galaxy formation that mergers are too common in LCDM to explain the prominence of thin disk-dominated galaxies in the local universe. In my dissertation, I analyze merger histories of dark matter halos from high resolution N-body simulations and compare dark halo merger statistics to the observable properties of galaxies, in order to study the implications of cosmologically motivated merger histories. I use empirical relations between a galaxy's dark matter halo mass, stellar mass, and cold gas mass to investigate these merger histories in the context of galaxy evolution, focusing on a dark matter mass regime within an order of magnitude of the Milky Way.
The principle results of this dissertation may be summarized as follows. Firstly, 70% of Milky Way-size halos have accreted an object with more than twice the mass of the Milky-Way's disk in the past 10 Gyr. To meet the observed fraction of disk dominated galaxies, mergers of this size must not always destroy galactic disks. Secondly, The merger rate of dark halos increases strongly with redshift. A simple 'universal' fitting formula describes these merger rates as a function of halo mass, merger mass ratio, and redshift. Thirdly, the fraction of halos have ever experienced a gas poor major merger roughly matches the observed early-type morphological fractions within the regime M = 10^11-13 Msun, providing a possible solution to disk survivability, if gas rich mergers do not destroy disk-dominated morphologies. Fourthly, because the mapping between dark matter halo mass and galaxy stellar mass (or baryonic mass) is a non-trivial function, it is important to distinguish between definitions of a merger "mass ratio" that use dark matter, stellar, or galaxy baryonic masses as a means of comparison. For example, major dark matter mergers in smaller galaxies (M_DM < 10^11 Msun) typically corresponds to very minor stellar mergers. Finally, we use a higher resolution simulation to focus on the substructure of Milky Way-sized galaxies. We predict a population of "stealth" galaxies: low-luminosity dwarf galaxies in the halo of the Milky Way with surface brightnesses so low they are unobservable with current methods.

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