The Influence of Dark and Ordinary Matter Physics on Galaxy Formation

Abstract

Overwhelming observational evidence suggests that 85$\%$ of all the matter in the universe is dark matter (DM), a particle whose microscopic properties remain poorly constrained over many orders of magnitude. The current, widely assumed paradigm of a collisionless, cold DM (CDM) and dark energy cosmology called $\Lambda$CDM has proven to be very successful on large scales. Yet, observed galaxies are generally less dense than simple CDM-only predictions, and while CDM is often assumed to be a single, collisionless particle species, there are no Standard Model particles that are similarly collisionless. These discrepancies suggest small-scale problems for the $\Lambda$CDM paradigm and have ignited the astrophysical community to consider models of DM which abandon the collisionless assumption. This thesis details the use of hydrodynamic simulations, analytic and numerical methods, and observations to examine the fundamental nature of DM by asking how altering its microscopic properties can influence the largest scales, with an emphasis on galaxy formation. In particular, using analytic and numerical minimization methods we show that if DM is charged, collective plasma processes may dominate momentum exchange over direct, short-range particle collisions. Using cosmological hydrodynamic simulations, we find that self-interacting DM with an interaction cross-section of $\sigma_{\rm SI} = 1 $cm$^2$/g delays supermassive black hole growth through mergers by billions of years compared to CDM growth. With the same simulations, we show slow accretion of cold clumps through the circumgalactic medium and onto galaxies is an important process that fuels star formation, independent of background DM.