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

TL;DR

This work dissects the drivers of cosmic reionization by coupling galaxy-scale physics to IGM evolution through state-of-the-art radiative transfer simulations. It demonstrates that SN feedback strongly modulates the UV luminosity function of early galaxies, with bursty star formation producing the largest UVLF variability, especially in low-mass halos, potentially addressing JWST’s bright-galaxy tension. It then shows that the ionizing spectra of sources—stars, X-ray binaries, and AGNs—imprint measurable differences in the hydrogen reionization timeline and IGM thermodynamics, modestly affecting Ly$\alpha$ forest statistics but notably altering temperature and proximity-zone properties. The helium reionization results highlight the pivotal role of quasars, explore how JWST-era QLF updates influence the He II ionization history, and reveal a generally delayed He II reionization relative to older models, with residual uncertainties from sub-grid sinks. Finally, it introduces the 3.5 cm hyperfine transition of $^3$He$^+$ as a novel observational probe of He II reionization, mapping its expected signal and cross-correlation with quasars, and outlining its prospects with future radio facilities like SKA. Collectively, the thesis integrates galaxy formation, radiative processes, and IGM evolution to produce a coherent, testable framework for the start and end of cosmic reionization and its observational signatures.

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.

Figures (52)

• Figure 1: Power spectrum of the temperature fluctuations of the CMB as measured by Planck satellite (solid data points). The solid line represents a fit based on a six-parameter $\Lambda$CDM cosmology. This figure is adapted from planck2016a.
• Figure 2: The panels display the large-scale structure of the galaxy distribution, with blue points representing observed data and red points representing simulation. The observed and simulated galaxy distributions show a strikingly similar pattern. This figure is adapted from springel2006.
• Figure 3: Virial temperatures and mass scales of dark matter halos that facilitate the formation of the first stars, primordial galaxies, and the most massive galaxies in the Universe. The cooling mechanism shifts from molecular hydrogen in smaller halos to atomic hydrogen in more massive halos, driving cosmic structure formation. The figure is taken from wise2019.
• Figure 4: Left panel: Relation between gas number density n and gas temperature T for a cloud of gas. Right panel: Number density of galaxies and halos as a function of respective mass showcasing the importance of feedbacks in shaping that in different mass regime. The figures are taken from laursen2023.
• Figure 5: Left panel: Redshift evolution of the comoving volume averaged ionizing emissivity per source type assuming either a constant UV escape fraction $f_{esc}$ = 15$\%$ (solid lines) or redshift-dependent $f_{esc}(z)$ (dashed lines). From top to bottom the sets of curves correspond to: stars (black), BHs (blue), XRBs (purple) and ISM (red). Right panel: Average SEDs for each source type, stars (solid lines), BHs (dashed), ISM (dotted) and XRBs (dash dot-dotted) as function of photon energy plotted at various redshifts (indicated by the line colour). The ionization thresholds for HI, HeI and HeII are plotted as vertical grey lines The figure is taken from Marius2020.