The Coevolution of Galaxies and Their Environments

Abstract

The circumgalactic medium (CGM), or ``halo gas", represents a major component of the baryon cycle of galaxies—-a component which remains poorly understood. Galaxies grow by accreting from this reservoir, and also return enriched material to this atmosphere through the action of galactic winds and feedback. Galaxies and their environments hence coevolve with time. In this dissertation, we use cosmological hydrodynamic simulations to study this relationship in detail. In Chapters 2-4, we present three complementary simulation studies, each of which maximizes feedback diversity, galaxy diversity, or CGM resolution, respectively. In Chapter 2, a high redshift population of galaxies are studied using a suite of cosmological simulations which feature various feedback variations, in order to identify which feedback parameters are most important in determining the global properties of the CGM. We find that the wind energy per unit outflow mass has the most significant effect on the enrichment of the CGM. If winds are strong, the IGM are enriched by very small galaxies at early times. Conversely, if winds are weak, metals are generally locked within the halo until the more vigorous AGN feedback engages at late times. In Chapter 3, we study the CGM of the low-redshift population of the Illustris simulation, with the goal of trying to reproduce observed trends in abundances of highly ionized oxygen (OVI) in the warm-hot phase around these galaxies. Most notably, we find that the observed OVI abundance disparity between star-forming and passive galaxies is reproduced in the simulation and is due to the action of AGN radio mode feedback. In Chapter 4, we employ extremely high resolution simulations which focus resolution specifically in the CGM to study the origin and production of cold ($T \sim 10^4$ K) CGM gas. We find that the presence of winds completely changes the nature of the cold phase. Without winds, the cold phase is mostly gas in the outer halo which has not yet virialized; when winds are turned on, the cold phase is dominated by material which has cycled through the galaxy, been shock-heated in an outflowing wind, and rapidly cooled back into the cold phase. This finding has significant implications for how to interpret recent observational findings that cold CGM clouds may be ubiquitous around all galaxies, irrespective of mass or star-formation rate.