Investigating Trends in the CGM and IGM During Reionization

The Epoch of Reionization (EoR) was a period in the Universe’s history wherein the wholly neutral IGM was ionized by the rapidly evolving ionizing background. Due to the high opacity of the IGM during this epoch, studying this period is difficult, yet it could reveal much about the traits of early galaxies. In this dissertation work, we demonstrate that reionization may also be studied through analysis of metal line absorption systems such as those seen in quasar spectra, arising from enriched gas in the CGM and IGM, complementary to methods focused solely on neutral hydrogen. To accomplish this, we use cosmological simulations of the EoR, including on-the-fly multi-frequency radiative transfer, using the Technicolor Dawn code. Through generation of mock quasar sightlines and simulated spectra, we analyze metrics of metal absorption and other gas parameters to illuminate their connection to the process of reionization. Additionally, by incorporating several stellar population synthesis models, including one with binary stars, we examine how differing stellar emissivities impact the timing of reionization and the preferred ionization states of metals in the enriched CGM and IGM. We find that the ionization states of common absorbing transitions are sensitive to the amplitude and spectral shape of the ionizing background, suggesting that careful observations of these systems will place constraints on the nature of the primary contributors to reionization, particularly whether quasars could have played a major part. Further, we determine the redshift evolution of metals is dependent on the ionizing background and, through abundance ratios of neutral oxygen and silicon, can be directly tied to the neutral fraction of hydrogen, assisting in studies of the timing of reionization. Finally, we show that while the harder spectra of binary stars can affect elemental ionization states and lead to an accelerated reionization history, it can also lead to decreased star formation rate density through gas heating, and subsequently decreased levels of enrichment in the CGM and IGM. This work reveals a complex interplay between several simulation parameters, and also highlights the usefulness of metal line studies at z>5.