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 \ge 7 $ observations. Throughout the redshift range $7 \le z \le 15$ we identify "Pop III-bright" galaxies as those with at least 75% of their flux coming from Pop III stars. While less than 1% of galaxies brighter than $m_{\rm UV, AB} = 31.4$ mag are Pop III--bright in the range $7\leq z \leq8$, roughly 17% of such galaxies are Pop III--bright at $z=9$, immediately before reionization occurs in our simulation. Moving to $z=10$, $m_{\rm UV, AB} = 31.4$ mag corresponds to larger, more luminous galaxies and the Pop III-bright 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 III-bright regardless of magnitude. While $m_{\rm UV, AB} = 31.4$ mag galaxies are extremely rare during this epoch, we find that 13% of galaxies at $z = 14$ are Pop III-bright with $m_{\rm UV, AB} \le 33$ mag, an intrisic magnitude within reach of the James Webb Space Telescope using lensing. Thus, we predict that the best redshift to search for luminous Pop III--bright galaxies is just before reionization, while lensing surveys for fainter galaxies should push to the highest redshifts possible.