This thesis covers three projects, all centred around using computational simulations to theoretically model the intergalactic medium (IGM) and Lyman-α emitting (LAE) galaxies in the early universe. These galaxies are a key observational probe for understanding the period of the Universe’s history known as the Epoch of Reionization (EoR). In this thesis I explore:

(i) how we can use simulations to model the IGM transmission to LAEs during reionization, and what simulations reveal about the environmental dependence of this transmission. I use these simulations to compare the circumgalactic medium (CGM) and infall environments of bright LAEs residing in more massive host haloes with those of fainter LAEs residing in less massive haloes, and how their environments affect the visibility of LAE populations.

(ii) how to model the population statistics of LAEs and use them to constrain reionization. I implement a model for populating LAEs within dark matter haloes of numerical simulations, which can be used to explore the effect of the IGM on the luminosity function and angular correlation function. By comparing the simulation predictions with current observations it is possible to explore which reionization scenarios are favoured or disfavoured.

(iii) how we can combine LAE and 21-cm observations to derive further constraints on reionization. Using the simulations and modelling established in my first two projects, I explore the cross-correlation of LAEs with 21-cm emission as a further observational statistic for learning about reionization. To compare to observational results I forecast survey sensitivities for a selection of different current and upcoming programs.

Throughout these projects I look in particular at late reionization scenarios, the evidence for which has been growing with recent observational results. By comparing our theoretical late reionization results to those of early reionization scenarios, we can also better understand how to distinguish these possibilities observationally.