Colloquium (Ph.D. dissertation defense): Caitlin Doughty

April 7, 2021 @ 12:00 pm – 1:00 pm

Implications of z>5 metal absorption for the sources and progression of hydrogen reionization

Caitlin Doughty, NMSU

The epoch of reionization occurred over 12 billion years ago, and saw the neutral hydrogen in the intergalactic medium transition from neutral to ionized. This period is studied using emission from galaxies or through hydrogen absorption as seen in quasar spectra, with an eye toward constraining the evolution of the ionization fraction and determining the physical sources of ionizing photons. However, studies of emission require assumptions about the abundance of ionizing sources, and hydrogen absorption quickly becomes saturated due to its high column density. In this dissertation, we present numerical studies of an alternative probe, ionized metals, and explore their effectiveness in determining the progression and sources of reionization. To accomplish this, we use cosmological simulations and generate mock spectra of metal ions, and perform extensive comparison to observations. We find that the ratios of abundances of several tracer ions are sensitive to the sources of reionization, but that some ratios are more useful than others in distinguishing between ionizing sources. Further, the co-occurrence of Si IV and C IV differs significantly between galaxy- and quasar-dominated background models. We analyze the evolution of a similar metal transition to neutral hydrogen, O I. We find that neutral oxygen undergoes rapid evolution near the end of reionization, being preferentially ionized in low column density systems with higher impact parameters. We find the ratio of comoving densities in O I and silicon to be a good tracer of the mass-weighted neutral hydrogen fraction, and thus a tracer of the progression of reionization in denser circumgalactic gas. Finally, we investigate the impact of high fractions of binary stars on reionization and metal ions to discover whether their effects should be observable. We find that binary stars’ harder spectra cause an accelerated H I reionization, but cause photoheating that reduces galaxies’ ability to form stars and drive early He II reionization. Further, the harder background increases (decreases) the incidence of high (low) ionization ions in mock spectra. In sum, we demonstrate that metal ions can serve as probes of reionization, and provide possible avenues of exploration for future observational studies.

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