UC Berkeley

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.