Cosmic Reionization With JWST: The Growth of Ionized Bubbles and the Nature of Early Galaxies

Abstract

Over the past two decades, substantial progress has been made in understanding both the epoch of reionization and the nature of galaxies within the first billion years of cosmic history.Observations with various independent probes have suggested a consistent picture that the reionization of intergalactic hydrogen was well underway at $z>7$ and largely completed by $z\sim6$. Large samples of early galaxies have been routinely detected and spectroscopically confirmed now out to redshifts beyond 10. However, important questions remain regarding how reionization proceeded spatially and the physical nature of the early galaxies. In this dissertation, I first take steps to investigate the timeline of reionization and the emergence of ionized structures in the early universe, leveraging Ly$\alpha$ observations from JWST/NIRSpec. By directly measuring the Ly$\alpha$ escape fraction as a function of redshift, which has been challenging to estimate in the ground-based era, I find a decline of the typical escape fraction at $z>7$, as expected if the IGM becomes significantly neutral towards higher redshifts. By characterizing the large scale environments of strong Ly$\alpha$ detections at $z>7$, I find that they are almost uniformly located within galaxy overdensities, consistent with these overdense regions tracing large ($>$1~pMpc) ionized bubbles. I show that this is in contrast to what is seen at $z\sim5\mbox{--}6$, likely reflecting the increasing importance of overdensities in carving out ionized bubbles to facilitate Ly$\alpha$ transmission at earlier stages of reionization. Second, using deep high-resolution NIRCam imaging, I spatially resolve the internal structures of the most luminous reionization era galaxies, finding they frequently comprise multiple bright star-forming clumps. These clumps dominate the total light, yet with clear variations of the stellar population ages, likely representing an active phase of early galaxy assembly. I extend this spatially resolved analysis to a sample of local metal-poor dwarf galaxies with spectroscopic properties comparable to those seen in the reionization era. My results reveal a sequence in ionized gas morphology as a function of the typical age of the underlying star clusters, offering new insights into interpreting observations of early galaxies. I conclude this thesis by outlining future opportunities for systematically characterizing the growth of ionized bubbles during reionization and the assembly of early galaxies.