Recent observations suggest we do not fully understand how galaxies form at high redshifts. We use \texttt{Enzo} and \texttt{Enzo-E} cosmological simulations to study the transitional period between primordial star formation and the buildup of metal-enriched protogalaxies. We focus on metal enrichment, accretion, cooling, chemistry, and virialization of gas within dark matter halos before the Epoch of Reionization.
In Chapter \ref{ch:external_enrichment}, we analyze an \texttt{Enzo} cosmological simulation and find that the dominant mode of chemical enrichment in minihalos is the accretion of metals sourced from supernovae occurring outside of the virial radius. We find that stellar populations that form following external enrichment have low metallicity.
In Chapter \ref{ch:methodology}, we document the code development and testing efforts that were made to prepare \texttt{Enzo-E}, \texttt{Enzo}'s highly scalable successor, for use in running large-scale cosmological simulations. Necessary developments include primordial and metal-enriched star formation and feedback models, as well as a multigroup M1 closure radiative transfer solver. Comparisons between identical \texttt{Enzo} and \texttt{Enzo-E} simulations are also made.
In Chapter \ref{ch:phase1}, we examine $\sim12,000$ dark matter halos in the absence of metal-enriched star formation and feedback at $z\simeq12$ in \texttt{Enzo-E} simulations. We find that 16\% of halos are metal-enriched by $z\simeq12$, and that metals in the most massive halo are sourced from at least 20 separate primordial stellar populations. Without heating from stellar feedback, gas virialization is supported by nonthermal bulk flows, and atomic cooling is inactive. We find that H$_2$ is the dominant coolant at the center of halos with $10^6\,\,M_\odot
In Chapter \ref{ch:phase2}, we include metal-enriched star formation and feedback in our \texttt{Enzo-E} simulations and follow the evolution of a single halo under different feedback prescriptions until $z\simeq18$. We find that radiative feedback controls the early star formation history of the protogalaxy through photoevaporative flows. H$_2$ remains the dominant coolant in the presence of stellar feedback due to H$_2$ formation in relic HII regions. Finally, we find that the virial temperature does not accurately describe the thermal state of the gas at any point in the halo's early lifetime.
