Understanding galaxy formation is an enduring questions in astrophysics. Galaxies are systems rich in interesting physical processes, and the vast range of times and environments in which galaxies form and evolve offer a wealth of challenges. One of the primary driving forces in galaxies is the interaction of stellar populations with their surroundings. The nature of this interaction drives the evolution of both components, and the resulting behavior has a profound impact on the observable universe. In this dissertation I discuss the results of modeling this interaction in a variety of contexts using semi-analytic methods in conjunction with high-performance numerical simulations to bridge the huge dynamic ranges spanned by these processes. With these techniques I explore the environments in which Population III stars form, studying the transition to chemically-enriched star formation, and quantifying the changing environment and assembly history of the dark matter halos which host Population III stars in a universe of increasingly chemical complexity. Chemical enrichment in high redshift proto-galaxies is investigated by coupling semi-analytic models of star formation and feedback to cosmological N-body simulations. The resulting elemental abundance ratios are compared to those observed in metal-poor stars and satellite systems of the Milky Way, with the comparison constraining the nature of Population III stars, galaxy formation at high redshift, and the transition from metal-free to chemically enriched star formation. Ensembles of semi-analytic models representing internal galactic processes are used to develop a new formalism for representing galaxies in cosmological simulations of galaxy clusters. This method is used to investigate galaxy formation in a cluster environment and the interaction between cluster galaxies and the intracluster medium. The interaction between stellar populations in disk galaxies and the diffuse circumgalactic medium is studied in simulations of idealized disk galaxies. The interplay of stellar feedback and the development of multiphase gas in the circumgalactic medium, and in turn the influence of this multiphase gas falling back onto the stellar disk is investigated.
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