Statistical Signatures of Cosmic Dawn Galaxies

Radiation from the first stars, black holes and galaxies heated and ionized the pervasive neutral hydrogen in the intergalactic medium (IGM), culminating in the final phase change of our Universe: the Epoch of Reionization (EoR). This important epoch is the observational frontier, with current telescopes providing limited data spanning the first billion years of our Universe. However, the EoR is an inhomogeneous and complicated process with current telescopes being able to only detect the very brightest of EoR galaxies. As a result, there are many open questions, including: When did the EoR happen? What are the sources driving it? What were the properties of those sources? This thesis focuses on the inference techniques and modeling improvements that are key to answering these questions. Several large-scale probes can be used for EoR inference, including the Lyman-α forest in high-redshift QSO observations, CMB optical depth, galaxy observations including the galaxy luminosity function, Lyman-$alpha$ observations, and the patchy kinetic Sunyaev-Zel’dovich effect (a secondary anisotropy of the CMB sourced by free electrons and the velocity field during EoR). We combine these probes using Bayesian inference and semi-empirical scaling relations to infer a late-ending EoR. However, we demonstrate that some results can also depend on the galaxy-to-galaxy scatter around galaxy scaling relations. We investigate how such stochasticity in astrophysical relations impacts various quantities that are key for EoR science. We find that the scatter in the star-formation relation is key for modeling emissivities, UV luminosity functions, and the global EoR history. This stochasticity can also be learned from data, and we present methods to do so. Answering questions about what sources drove the EoR is intrinsically linked to the patchy nature of the process, as reflected in the morphology of the ionization field. We introduce a method to do this through the damping wing absorption of neutral hydrogen in the observed Lyman alpha emission lines of galaxies, showing that it will allow us to infer ionized bubbles surrounding groups of JWST-observed galaxies. By combining inferences across global and local scales, current and future telescopes are poised to uncover the details of the EoR and the first galaxies that drove it.