REX, the reionization explorer: science and mission overview

REX is a NASA Astrophysics Small Explorer Mission concept to chart the history of cosmic dawn in unprecedented detail in space and time. REX will identify very young galaxies and black holes by means of their powerful Lyman alpha (Lyα) line emission using about 10 narrow-bandpass filters covering about 100 square degrees. The strong line emission identifies samples of the most actively star-forming early galaxies, believed to be the drivers of reionization. Moreover, mapping the distribution and properties of the Lyman alpha emitting population will reveal the distribution of ionized and neutral gas, because neutral gas scatters Lyman alpha light, rendering them difficult to detect. REX will use an 0.5-1m telescope and 1 square degree field of view, tiled with HgCdTe detectors with development heritage from the Nancy Grace Roman Space Telescope. Its large, flexible filter complement will be used in a point-and-stare mode to identify Lyα emitting galaxies at a range of discrete redshift slices spanning the reionization era. In addition to its core reionization surveys, REX brings a new capability of tracing gas emission over large scales at the peak of star and black formation era. We will find millions of the youngest, least massive galaxies in epochs spanning the most active growth period of the universe. Applications will include ionized gas in nearby and distant galaxies, active galactic nuclei, and galaxy clusters. In summary, the REX survey will have the sensitivity and the area coverage to find the sites of earliest galaxy formation and will have the pixel size to enable good localization for follow up of individual galaxies with JWST and future telescopes.