Illuminating Cosmic Reionization with Galaxies in the First Billion Years

Abstract

In the last two decades, observations with state of the art optical and near-infrared telescopes have greatly advanced our understanding of the Epoch of Reionization. Measurements based on quasar spectra, the cosmic microwave background, and observations of Lyman-alpha (Lyα) emission in early galaxies have constrained the timeline of reionization, placing the conclusion at z~5.3, while reionization is still ongoing at z~7-8. However, though we have made significant progress understanding the volume-averaged timeline of reionization, many questions remain open about the details of the reionization process and the nature of the galaxies that produced the ionizing photons that reionized the Universe. In this dissertation, I explore both of these topics, with the goal of developing a holistic picture of how galaxies drove the reionization process. Using the most comprehensive extragalactic imaging survey available to date from the James Webb Space Telescope (JWST), I quantify the abundance of galaxies at z~9-16 by measuring the rest-UV continuum luminosity function, and examine implications for early star formation processes and the ionizing photon budget available to reionize the Universe. Next, I examine the properties of galaxies at z~7 and z~9 in more detail, focusing on characterizing the star formation histories of the population and the evolution thereof over cosmic time, in order to better understand how early galaxies formed stars and produced ionizing photons. Then, I turn towards directly linking the reionization process to specific populations of galaxies that created early, large ionized regions, first leveraging photometric signatures of overdensities of galaxies to identify candidate ionized bubbles, then following these candidates up with spectroscopy to measure Lyα emission and constrain ionized bubbles on local scales. Finally, I discuss future prospects for furthering our understanding of the star-forming and ionizing properties of early galaxies using deep near-infrared observations, alongside direct constraints on the topology of the reionization process from Lyα emission.