First Galaxies: Exploring the Reionization Epoch

Executive Summary: Studies of the nature of the first galaxies, and of the processes that led to their assembly and buildup, are of compelling importance. Not only did the first generation of galaxies likely reionize the universe, but they also were the seeds for the diverse population of today’s galaxies and the massive black holes that reside in them. Star formation in the first galaxies was likely very different than at later times, with different feedback processes dominating and early galaxies hosting an early generation of primordial stars. The challenges of charactering the fascinating physical processes at work during the first hundreds of millions of years to z∼6-7 are considerable. Characterizing these galaxies in detail with observations is difficult due to their small sizes, extreme faintness and the need for sophisticated IR instruments for detection and measurement. Yet such observations are needed if our models are to be realistic. With a 30-m class GSMT, we should have the necessary spatial and spectral resolution to push to the low luminosities needed to characterize their contribution to reionization and to do detailed studies of their sizes, structure and morphologies (Figure 1). The synergies of GSMT with ALMA and JWST would enhance this field greatly.