Studying reionization with the newest space telescopes

Abstract

Reionization is the process in which the universe's intergalactic medium (IGM) went from neutral to ionized. This is the last major phase change of the universe and it is driven by, and influences, the first galaxies' formation. Recent efforts have developed observational methods to constrain the timeline of reionization. Large reionization simulations have also been developed to aid with interpreting the observations. Lyman-alpha is extremely sensitive to neutral hydrogen making it an ideal probe to determine the ionization state of the IGM. High-z galaxies which emit Lyman-alpha may have this emission completely scattered out of our line-of-sight if there is residual neutral gas in their local intergalactic medium. Thus, if we observe evolution in the number of Lyman-alpha Emitters (LAEs) or the strength of their emission, it is likely that we are beginning to see reionization occur. However, these observations are complicated by the fact that bright LAEs are intrinsically rare objects and that reionization is expected to be a patchy process, both of which make them prone to significant cosmic variance. We develop an LAE simulation built on a realistic reionization simulation to determine the best survey strategies to mitigate the effects of this stochasticity. We use our LAE simulation and James Webb Space Telescope (JWST) observations of GN-z11 at z=10.6 to place one of the highest redshift constraints on the timeline of reionization to date. Taking into account the newest JWST observations of excessive high-z UV bright galaxies, we find that our inferred upper limit on the global neutral fraction at z=10.6 favors scenarios where many faint galaxies drive reionization, rather than bright galaxies. Euclid has the ability to study reionization by detecting LAEs in its deep fields. The observational parameters for the deep fields were not set until recently, guided by work in this dissertation. With newly developed simulation software, we find observation strategies that allow Euclid to meet its strict requirements for the deep fields. We use the software to determine the optimal ratio of blue-to-red grism observations in the deep fields with respect to the mission requirements and legacy science.