UC Davis

The \textit{Epoch of Reionization} marks the period following the Dark Ages, the era beginning 400,000 years after the Big Bang in which neutral hydrogen permeated the Universe. Reionization is defined as the period of cosmic history in which this hydrogen went from being neutral to ionized, ending at around redshift $z \sim 6$, or about 1 billion years after the Big Bang. This period marks the last major phase change of hydrogen in the intergalactic medium. It was also a time of significant galaxy and structure formation, when the first sources of light emerged. These nascent galaxies were likely major contributors to reionization, emitting large amounts of high energy photons which escaped into the intergalactic medium and began to ionize the vast sea of neutral hydrogen. However, beyond these facts, there are still many open questions surrounding the Epoch of Reionization. Some of the remaining uncertainties concern how quickly the transition occurred, what the key drivers were in the process, and what their physical properties are.

This dissertation aims to address some of these lingering questions. In this work, I use a large sample of characteristically faint, gravitationally lensed, high-redshift galaxies in order to constrain both the timeline of reionization and the physical properties of the ionizing sources. The galaxies used in this sample have deep photometric data in multiple bands from the \textit{Hubble Space Telescope} and \textit{Spitzer Space Telescope}, as well as followup spectroscopy from the \textit{Keck Observatory}. The high quality of photometric data provides good constraints on the redshift and physical properties of galaxies in our sample, even if they lack spectroscopic confirmation. The sample is comprised of $\sim$ 250 Lyman Break galaxy candidates within the redshift range $5 < z < 8.2$, spanning the heart and tail ends of the reionization era. Galaxy candidates for spectroscopic follow-up were chosen after multiband photometric observations were completed. These were selected via the Lyman Break technique and using constraints on the redshift probability distribution created from each galaxy's photometry. Follow-up spectroscopic observations resulted in 38 spectroscopically confirmed galaxies via detection of the Lyman-$\alpha$ (Ly$\alpha$, 1216\AA) line, and constraints on Ly$\alpha$ strength for the rest of the sample. For all analyses done in this dissertation, I use these $\sim$ 250 galaxies lying in the redshift range $5 < z < 8.2$ in order to constrain both the timeline of reionization and the stellar and UV properties of galaxies in this epoch.

I begin in Chapter 2 with a study on the timeline of reionization using Ly$\alpha$ emission properties from galaxies during and directly after the process was complete. In this work, I compare the prevalence and strength of Ly$\alpha$ emission from galaxies within the Epoch of Reionization ($z\sim 6.7$ and $z\sim 7.6$) and when reionization was mostly, if not entirely, complete ($z\sim 6$). I compare the UV luminosities and $\beta$ slopes of the samples at the different redshifts and show that the distributions are similar for both properties. As the galaxies all have similar UV properties, and therefore are likely at similar states of interstellar medium evolution, we attribute any difference in the Ly$\alpha$ equivalent width distributions to the evolving opacity of hydrogen in the intergalactic medium. The results of this study, which are consistent with other works, suggest a rapid and fairly late reionization scenario. This work is published in the \textit{Monthly Notices of the Royal Astronomical Society}, \citep{bolan_inferring_2022}.

In Chapter 3, I present an analysis of the physical properties of the galaxy sample as well as the results of a search for CIII] emission in confirmed Ly$\alpha$ emitters, which can provide systemic redshifts as well as a basis on which to infer metallicities and ionization parameters. For each of the galaxies, I calculate UV luminosity and $\beta$ slope, the slope in the UV spectrum of a galaxy redward of Ly$\alpha$ emission, from photometry as well as estimate stellar mass, star formation rate, specific star formation rate, and mass-weighted age via spectral energy density fitting. For galaxies with Ly$\alpha$ emission, I look at the equivalent width (EW) of the emission line as a function of all of these properties to see if there are any correlations. I also compare the distributions of each of these properties for Ly$\alpha$ emitters against nonemitters to see if there are significant physical differences between these groups of galaxies. I find no statistically significant relationships between Ly$\alpha$ EW and any physical properties, nor any significant difference between the sample of Ly$\alpha$ emitters and nonemitters.

In Chapter 4, I provide a summary. Using a sample of gravitationally lensed, intrinsically faint, high-redshift galaxies, I infer a timeline of cosmic reionziation and characterize the physical properties of typical galaxies from the era. These analyses are especially important as we enter an era of massive space and ground based observatories, such as the \textit{James Webb Space Telescope}, the \textit{European Extremely Large Telescope}, the \textit{Thirty Meter Telescope}, and the \textit{Giant Magellan Telescope}. The work done in this dissertation improves the of knowledge on high-redshift galaxies to guide future observations and surveys. With deep data on large samples of early galaxies from these observatories, a detailed timeline of reionization can be even further constrained, as well as the properties of the main drivers of the process.