NSF-BSF: Galaxies in the Reionization Era

The age of the universe is about 13.8 billion years. The first galaxies in the universe are believed to have formed when the universe was about 100-200 million years old, from clouds of hydrogen that collapsed under gravity. From various observations we know that by the time the universe was ~1 billion years old, the neutral hydrogen in the early universe has become re—ionized: it absorbed large amounts of energetic ultraviolet photons that made essentially all hydrogen atoms lose their electrons. However, it is still unclear how this process of reionization occurred — whether the first galaxies produce strong enough ultraviolet, ionizing radiation to drive it, and what is the main source of energetic, ionizing photons in the first generations of galaxies. Can primordial stars account for the needed radiation? are massive and active black holes needed to explain these?

Recent spectroscopic studies have demonstrated that some galaxies in the reionization era exhibit ultraviolet metal emission lines that indicate a hard ionizing spectrum, so that galaxies may have contributed more to reionization than previously thought. These efforts have also found that bright, high—redshift (2 E, 6; universe age of ~1 billion years) galaxies with strong optical emission lines can show Lyman—alpha emission — only seen when the vicinity of the galaxy is largely ionized — out to redshift z 3 9 (universe age of ~ 500 million years), thus indicating these reside in early reionized bubbles.

However, the faint nature of the vast majority of these early sources has long made detailed characterization prohibitive. Fortunately, we are now on the cusp of a golden period of early galaxy spectroscopy. One example is the next—generation, James Webb Space Telescope (JWST), aimed to reach the first generation of galaxies. However, because they are rare, massive galaxies will not feature in the pencil—beam JWST surveys that are planned in the near—futurc. Our proposal demonstrates the important role ground—based facilities will play in driving our understanding of reionization and the build—up of early galaxies. The proposed study builds on an Atacama Large Millimeter Array (ALMA) large program targeting a statistical sample of massive reionization—era galaxies. These are among the brightest systems known at this epoch and likely probe the earliest sites of structure formation. We are augmenting the ALMA observations with a multi—year spectroscopic survey targeting Lyman—alpha and fainter diagnostic lines in the rest—frame ultraviolet, providing detailed insight into the gas and stars in these systems. Our wide—area spectroscopic survey will characterize the physical nature of these massive galaxies and the progress of reionization around them, identifying the largest ionized bubbles within the first billion years of cosmic time.