Abstract

The history of reionization in the early universe remains substantially unknown. This is, in part, due to a decrease in observable Lya lines at z> 6 -- a likely consequence of neutral intergalactic gas prior to the completion of the reionization process. While the end of reionization is a frequent target of observations, how this process begins and what causes it are presently unconstrained. One scenario allows reionization to start late and complete rapidly, while another suggests a smooth temporal evolution of reionization. The key to unlocking this mystery (and the ionizing sources responsible) lies in each scenario's predictions for the ionized fraction at z~9, where the results differ dramatically (~20% vs. ~60%). My recent MOSFIRE program has sought Lya emission at this epoch to distinguish between these two possibilities. In this talk, I will discuss the process I established using Hubble Space Telescope grism data to remove contamination and detect a faint Lya emission line at z=7.451 (Larson et al. 2018), whose high-EW Lya line held implications for large ionized regions well into the reionization era. Building from that, I will detail a similar method I implemented using deep Keck/MOSFIRE spectroscopy alongside a rigorous data reduction process to detect Lya at z=8.661 in the EGS field (Larson et al. 2022). This new galaxy lies physically close (3.5 pMpc) to another galaxy, with Lya detected at z=8.684 (Zitrin et al. 2015). These two spectroscopic sources and 3 nearby photometric sources might suggest the highest-redshift overdensity in the reionization era -- indicating the existence of > 1 Mpc ionized bubbles as early as 500 Myr after the Big Bang and shedding light on galaxy growth in the early universe. Our Cosmic Evolution Early Release Science (CEERS) SurveyJWST observations have revealed an accreting supermassive black hole in the brighter of these two EGS sources. This galaxy, CEERS_1019, may be a progenitor of massive z > 6 quasars, a key phase of the growth and formation of black holes in the early Universe, and previously unstudied (Larson et al. 2023). In preparation for the follow-up observations with JWST, I created a set of SED Templates optimized for the young, metal-poor, and highly-ionizing galaxies we would be studying in detail for the first time and whose colors were lacking in traditional galaxy models (Larson et al. 2022b). Future work with JWST will further expand upon our ability to quantify the impact these galaxies had on the reionization of the IGM around them and start to study them in detail while also placing them in the larger context of the Universe during this epoch with the plethora of early galaxy surveys.