Abstract

The reionization of the intergalactic medium (IGM) marks the time in the early universe when the first stars and galaxies began to affect the universe around them, as during this last major phase transition high-energy ultraviolet photons from these objects ionized the gas in the IGM, and it remains ionized to the present day.  Studying reionization is a key frontier in observational cosmology, as it can therefore provide key insights into the formation and evolution of galaxies in the early universe.  As Lyman-alpha photons are resonantly scattered by neutral hydrogen in the IGM, an analysis of this line can be used to trace the existence of neutral hydrogen in the IGM at different points in the history of the universe (i.e., when the IGM becomes neutral, we should stop seeing these photons, as they are scattered out of our line-of-sight).  The work in this dissertation focuses on completing a spectroscopic survey of galaxies in the early universe, to measure the Lyman-alpha equivalent width (EW) distribution into the epoch of reionization and investigate the evolution of the IGM during reionization, pinning down the late time-evolution of reionization.  To measure the Lyman-alpha EW distribution in the reionization era, we utilize deep spectroscopic observations of candidate galaxies from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) using both the DEIMOS (optical) and MOSFIRE (near-infrared; NIR) spectrographs on the Keck telescopes.  Our large spectroscopic dataset compiled with both Keck telescope observations represent the deepest and most complete spectroscopic survey for galaxies in the epoch of reionization. We study the Lyman-alpha emission strength through constraining the Lyman-alpha equivalent width (EW) distribution with our spectroscopic dataset by constructing detailed simulations of mock emission lines, accounting for the observational conditions (e.g., exposure time, wavelength coverage, and sky emission) and galaxy photometric redshift probability distribution functions. The measurements of the EW distribution with the detected Lyman-alpha emission lines from our DEIMOS and MOSFIRE observations provide additional evidence that the Lyman-alpha EW distribution declines at z > 6, suggesting an increasing fraction of neutral hydrogen in the IGM.

Thanks to plenty of high-quality observational data from space telescopes such as the Hubble Space Telescope and the Spitzer Space Telescope, observational studies of the evolution of galaxies in the early universe have been performed over the past decade. Those studies have revealed statistical trends of the star formation history and the evolution of those galaxies over the cosmic time. However, since the star formation happens in a complex way, an analysis of the integrated properties of galaxies is not enough to grasp the physical processes governing the star formation and the growth of galaxies. The advent of a spatially resolved study of each individual galaxy has provided us an excellent approach to explore star formation processes inside a galaxy and to examine the inside-out / outside-in growth scenarios of galaxies. We perform a spatially resolved study of galaxies in the early universe at z≳4 using the CANDELS Survey and HAWK-I UDS GOODS (HUGS) survey data. We estimate stellar mass, star formation rate, and dust extinction for galaxy inner and outer regions via spatially resolved spectral energy distribution fitting based on a Markov Chain Monte Carlo algorithm. By comparing specific star formation rates (sSFRs) between inner and outer parts of the galaxies we find that the majority of galaxies with high central mass densities show evidence for a preferentially lower sSFR in their centers than in their outer regions, indicative of reduced sSFRs in their central regions.