Abstract
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, where moderately luminous galaxies dominate the ionizing photon budget. Another scenario allows a smooth temporal evolution of reionization, where very faint galaxies, common at early times, dominate the photon budget. These models differ drastically in their predictions for the ionized fraction at z~9 (~20% vs. ~60%), yet the galaxies in this epoch have not been directly confirmed by spectroscopy. We propose to send the first probe into this era by searching for Ly? emission from a new sample of 11 photometrically selected galaxies at z~9-10. Simultaneously, we will measure redshifts for an additional ~50 galaxies at z~5-8 from other UV emission lines (CIII] and CIV) to confirm galaxies in this era and study their physical conditions. Due to its resonant scattering properties, Ly? has been a standard tool to probe the end of reionization at z~6-7, with many studies finding a lack of detectable Ly? lines at z~7. However, there are notable exceptions, with three Ly? detections at z=7.5-8.7, implying that ionized bubbles exist at z>8.5. We received two nights during the 2018A semester but lost one due to instrument malfunction. Our one night of data has revealed a tentative z=8.89 Ly? line which we have proposed to follow-up during the 2019A semester, as well as targeting 8 new z>9 galaxy candidates. Our proposed observations will push to yet higher redshifts - if we detect even a single galaxy in Ly? it will give both the first MOSFIRE line-based spectroscopic redshift at z>9, and provide evidence that the first ionized bubbles formed early on.
