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 $7 \le z \le 16$ we identify 'Pop III-bright' galaxies as those with at least 75% of their flux coming from Pop III stars. While less than 5% of galaxies brighter than $m_{\rm UV, AB} = 31.4$ mag are Pop III-bright between $7\leq z \leq8$, roughly a third of such galaxies are Pop III-bright at $z=9$, right before reionization occurs in our simulation. Moving to $z=10$, $m_{\rm UV, AB} = 31.4$ mag corresponds to more luminous galaxies and the Pop III-bright fraction falls off to 15%. Finally at the highest redshifts, a large fraction of all galaxies are Pop III-bright regardless of magnitude. While $m_{\rm UV, AB} = 31.4$ mag galaxies are likely not detectable during this epoch, we find 90% of galaxies at $z = 16$ are Pop III-bright with $m_{\rm UV, AB} \le 33$ mag, a lensed magnitude limit within reach of the James Webb Space Telescope. 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.