Modelling the Epoch of Reionization with Cosmological Radiative Transfer Simulations

Abstract

The epoch of reionization is the ionization of neutral hydrogen by UV photons emitted from the first sources. While its end is well constrained by Ly𝛼 forest observations, the timeline and contributing sources remain active research areas. In this thesis, I model reionization by post-processing a hydrodynamical simulation using aton-he, a radiative transfer code I developed for this purpose. The thesis is structured into four sections. The first section details the development of aton-he, focusing on implementing helium physics, active galactic nuclei (AGN) as ionizing sources, automation of the computational pipeline, and speed up of the code through parallelization. The second section focuses on the simulation setup, the model parameters, and the calibration methods. It further examines the impact of helium and multiple frequency bins on hydrogen reionization. The third section explores recent observations of a higher number density of faint AGN at 𝑧 ≳ 6. I implement faint AGN into the simulations and analyze the effects on reionization models, comparing AGN-only models to those where AGN contribute moderately alongside galaxies. Additionally, this section examines the viability of reionization being primarily driven by either faint or massive galaxies. The final section addresses new JWST observations of high-redshift (𝑧 ≳ 8) Ly𝛼 emitters. I explore two reionization models where the midpoint of reionization is shifted to higher redshifts, effectively initiating reionization earlier. These models predict the formation of ionized bubbles at higher redshifts, enabling the escape of Ly𝛼 photons. I discuss the simulation setup, observational constraints, and how bright high-redshift objects affect galaxy escape fractions. I also examine the clumping factor required to recover the reionization history. The thesis concludes by consolidating findings from these sections, highlighting their implications for reionization and its observational signatures.