Merging Galaxies and Dark Matter Halos

Mergers between distinct objects are a natural part of hierarchical structure formation. Mergers are also one of the most critical elements in the evolution of both galaxies and halos. I use high-resolution, cosmological volume simulations to explore galaxy and halo evolution and merging activity in a cosmological context, including environmental dependence, merger rates and dynamics, and how these processes in halos connect with those of galaxies.I first explore halo merging and evolution, focusing on its interplay with large-scale environment. While halo spatial clustering has been thought to depend only on mass, I ex- amine how spatial clustering depends on secondary parameters such as halo formation time, concentration, and recent merger history, a phenomenon known as "assembly bias". Next, I examine the extent to which close spatial pairs of objects can be used to predict mergers, finding limited utility to the pair-merger method arising from a competition between merger efficiency and completeness. I also explore the dependence of merging on environmental density, discovering that merging is less efficient in overdense environments. I then investigate how a massive galaxy/halo population at high redshift connects to a massive population of the same number density today, finding that scatter in mass growth and mergers between massive objects preclude a direct population mapping either forward or backward in time.In the latter part of this work, I explore the dynamics and mergers of galaxies in groups and clusters. I first examine the orbital distributions of satellite halos/galaxies at the time of infall onto a more massive host halo, finding that satellite orbits become more radial and penetrate deeper at higher host halo mass and higher redshift. I then track the evolution of galaxies in groups directly, examining the merger rates of galaxies over time and finding that galaxy mergers do not simply trace halo mergers. I also examine the small-scale environments of galaxy mergers, discovering that recently merged galaxies exhibit enhanced small-scale spatial clustering for a short time after a merger. Finally, by using abundance matching to assign stellar mass to subhalos, I explore the importance of merging vs. disruption processes for satellite galaxy evolution. I rigorously test the connection of galaxies to subhalos by comparing simulations against observed galaxy spatial clustering, satellite fractions, and cluster satellite luminosity functions, finding agreement in all cases