Tracing The Start and End of Cosmic Reionization - Exploring The Role of Ionizing Sources as drivers

Abstract: This thesis investigates the Epoch of Cosmic Reionization (EoR), a key period in the early Universe when the first luminous sources formed and their radiation transformed the intergalactic medium (IGM) from neutral to ionized. Understanding this process reveals how the first stars and galaxies formed, influenced their surroundings, and shaped large-scale structures. The work traces both hydrogen and helium reionization by identifying the sources that produced ionizing radiation and examining how this radiation affected the thermal and ionization history of the IGM. Using the SPICE cosmological radiation-hydrodynamics simulations, the first part analyzes the ultraviolet luminosity function (UVLF) of high-redshift galaxies. It shows that different supernova feedback models drive distinct UVLF variability across mass and redshift, potentially alleviating the bright-galaxy tension seen by JWST. The second part explores how spectra from ionizing sources i.e. single and binary stars, X-ray binaries, emission from the interstellar medium, and active galactic nuclei impact the timing, topology, and thermal history of hydrogen reionization and the Lyman-alpha forest. At lower redshifts, radiative-transfer simulations reveal how helium reionization driven by quasars progressed and how the timing and temperature of the IGM relate to quasar luminosity functions and He II Ly-alpha observations. Finally, predictions for the 3.5 cm hyperfine transition of singly ionized helium-3 offer a novel probe of the post-reionization IGM. Together, these studies combine galaxy formation, radiative processes, and IGM evolution to provide a coherent picture of cosmic reionization and its observable signatures.