Following the Cosmic Evolution of Pristine Gas. II. the Search for Pop III-bright Galaxies

Direct observational searches for Population III (Pop III) stars at high redshift are faced with the question of how to select the most promising targets for spectroscopic follow-up. To help answer this, we use a large-scale cosmological simulation, augmented with a new subgrid model that tracks the fraction of pristine gas, to follow the evolution of high-redshift galaxies and the Pop III stars they contain. We generate rest-frame ultraviolet (UV) luminosity functions for our galaxies and find that they are consistent with current z ≥ 7 observations. Throughout the redshift range 7 ≥ z ≥ 15, we identify "Pop IIIbright" galaxies as those with at least 75% of their flux coming from Pop III stars. While less than 1% of galaxies brighter than m_UV, AB = 31.4 mag are Pop IIIbright in the range 7 ≤ z ≤ 8, roughly 17% of such galaxies are Pop III-bright at z = 9, immediately before reionization occurs in our simulation. Moving to z = 10, m_UV, AB = 31.4 mag corresponds to larger, more luminous galaxies, and the Pop IIIbright fraction falls off to 5%. Finally, at the highest redshifts, a large fraction (29% at z = 14 and 41% at z = 15) of all galaxies are Pop IIIbright regardless of magnitude. While m_UV, AB = 31.4 mag galaxies are extremely rare during this epoch, we find that 13% of galaxies at z = 14 are Pop IIIbright with m_UV, AB ≤ 33 mag, a intrinsic magnitude within reach of the James Webb Space Telescope using lensing. Thus, we predict that the best redshift to search for luminous Pop IIIbright galaxies is just before reionization, while lensing surveys for fainter galaxies should push to the highest redshifts possible.