Exploring the Epoch of Reionization with Line-Intensity Mapping

Abstract

The epoch of reionization (EoR) stands as a key phase in the history of the Universe, where the neutral elements that pervaded the cosmos gradually became ionized. This process was driven by the photons emitted by the the first luminous sources and it strongly depends on their formation and evolution. Understanding the progression of the EoR means constraining the growth of structures and the interplay between galaxies and the intergalactic medium. In this work, we shed light on several aspects of this phenomenon, in order to get an insight of the general picture. <br /> This thesis is also motivated by the large amount of on-going and up-coming experiments that will finally unveil the core of the EoR. In particular, we make use of the line-intensity mapping (LIM) technique which allows us to study the large-scale structure of the Universe through the detection of the hyperfine transition in neutral hydrogen (HI) and single-ionized helium (HeII) atoms. <br /> The first project focuses on the connection between HI and the dark-matter counterpart within haloes, the HI-halo mass relation (HIHMR). We simulate mock HI intensity maps parametrizing the HIHMR and implementing systematic observational effects. We analyze the geometry and topology of the mock HI maps with the Minkowski functionals and we explore the dependence of the HIHMR on halo mass and galaxy properties. The results demonstrate the potential of LIM experiments, such as the Square Kilometre Array, to constrain the HIHMR. <br /> We then proceed to study helium reionization, an epoch for which we currently lack strong constraints regarding its drivers and endpoint. We investigate the imprint of different reionization models (early, driven by quasars at high redshift z > 5, and late, where active-galactic nuclei contribute from z = 5) on the power spectrum of HeII mock intensity maps, forecasting for radio surveys. We find that constraints on the timeline of the EoR and therefore on the ionizing sources can be achieved with such instruments. <br /> At last, we address the quenching of dwarf galaxies by investigating the impact of the reionization front on their star-formation rates (SFRs). Observations of the local Universe suggest different behaviours for galaxies with same features (e.g. mass, age). We assess the significance of galaxies' main characteristics using the Thesan simulations and we parametrize the probability of quenching with a simple analytical form. Our findings reveal how the ionizing radiation from early sources affects the gas reservoirs and the SFRs in these systems. We provide a valuable tool to connect the observations with the theoretical framework. <br /> With this thesis, we have explored several sides of the EoR. We have investigated the HI and the HeII evolution, the sources of reionization itself and its impact on galaxy evolution and we have provided forecasts for next generation instruments. We have paid attention to consider both sides of the phenomenon we study, observations and theory. Our findings refine the theoretical models on reionization processes and guide the interpretation of future observations, contributing to our understanding of the EoR and paving the way for further advancements.