Developing an Accurate Probe of the Galaxy-Halo Connection: Baryonic Effects, Small-Scale Galaxy Clustering, and Halo Model Extensions
Beltz-Mohrmann, Gillian Dora
Over the past 14 billion years, the Universe has evolved from a state of nearly uniform density into a complex web of structure. Slight perturbations in the initial density field grew over time via gravity, resulting in massive overdense regions of dark matter known as dark matter halos. These dark matter halos are the hosts in which galaxies form and reside. By comparing observations of galaxy clustering to our models of structure formation in the Universe, we can gain a better understanding of the physics that governed the initial conditions and subsequent evolution of the Universe. On large physical scales, where galaxies are simple tracers of the underlying dark matter distribution, our cosmological model is able to accurately predict the galaxy clustering that we observe. On small scales, however, galaxy clustering is affected both by our cosmological model and by the complex physics of galaxy formation and evolution, which is not well understood. Thus, it is difficult to test our cosmological model on small scales without first building an accurate picture of the connection between galaxies and the dark matter halos in which they reside. The goal of this dissertation is to develop an accurate probe of the galaxy-halo connection in order to improve both our understanding of galaxy formation and ultimately constrain our cosmological model. First, I use hydrodynamic simulations of galaxy formation to investigate the extent to which the assumptions of the standard halo model affect galaxy clustering statistics, and find that the halo model is unable to reproduce the galaxy clustering in hydrodynamic simulations. Next, I investigate the impact of baryonic physics on the halo mass function in three different hydrodynamic simulations, and find that not only is the effect on the mass function significant, but it varies widely from one simulation to the next. Finally, I add flexibility to the standard halo model to constrain the galaxy-halo connection in the Sloan Digital Sky Survey using a combination of small-scale galaxy clustering statistics. These results serve to improve our understanding of the galaxy-halo connection, which can ultimately be used to test our cosmological model on small scales.