Illuminating the Gas Cycling in and out of Galaxies

In this dissertation, we combine two different approaches — chemical evolution modeling and direct observations of the gas distributed around a galaxy — to study the gas cycling into and out of galaxies. The interplay of fundamental astrophysical processes that drive this baryon cycle, such as star formation and evolution, gas accretion from the cosmic web, and galactic winds, shapes the evolution of galaxies. These processes also determine the galactic gas enrichment history which is preserved in the stellar abundances of low mass stars that are still alive today. Therefore, stellar chemical abundance patterns can be used to indirectly illuminate the evolution of gas in and around galaxies. For the first part of this thesis, we build a flexible one-zone chemical evolution model to decode the star-by-star abundance patterns of nearby galaxies or the average abundance ratios derived from the integrated-light of distant galaxies. We characterize how and why these processes affect the observed stellar abundances and validate our code by modeling the stellar abundances of the Milky Way's geometrically-selected thick disk. We then utilize our chemical evolution model to help elucidate how the stellar populations of elliptical galaxies can simultaneously exhibit greater $\alpha$-enhancement and higher metallicities with increasing galaxy mass. Recent papers suggest that more massive ellipticals require more top-heavy IMFs. In contrast, we find that while a galaxy-mass dependent IMF is not precluded, the abundance patterns of these galaxies can also be explained if more massive ellipticals form their stars on shorter timescale and have outflows that are less enriched and/or are less efficiently driven by supernovae. In the second part of this thesis, we present direct observations of emission from a very bright enormous Lyman-$\alpha$ nebula (the Slug) that is illuminated by a nearby quasar at $z\sim2.3$. This is a rare opportunity to examine the physical properties of gas surrounding a galaxy that even extends beyond the halo into the intergalactic medium. We successfully detect H$\alpha$ emission from the Slug, indicating that the observed Ly$\alpha$ is produced \textemph{in situ} by hydrogen recombination and traces highly-ionized, optically-thick, dense clumps of cool gas