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- redshiftare 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 galaxiesand the Pop III stars they contain. We generate rest-frameultraviolet (UV) luminosity functionsfor our galaxiesand find that they are consistent with current $z \ge 7 $ observations. Throughout the redshiftrange $7 \le z \le 16$ we identify 'Pop III- bright' galaxiesas those with at least 75% of their flux coming from Pop III stars. While less than 5% of galaxiesbrighter than $m_{\rm UV, AB} = 31.4$ mag are Pop III- brightbetween $7\leq z \leq8$, roughly a third of such galaxiesare Pop III- brightat $z=9$, right before reionizationoccurs in our simulation. Moving to $z=10$, $m_{\rm UV, AB} = 31.4$ mag corresponds to more luminous galaxiesand the Pop III- brightfraction falls off to 15%. Finally at the highest redshifts, a large fraction of all galaxiesare Pop III- brightregardless of magnitude. While $m_{\rm UV, AB} = 31.4$ mag galaxiesare likely not detectable during this epoch, we find 90% of galaxiesat $z = 16$ are Pop III- brightwith $m_{\rm UV, AB} \le 33$ mag, a lensed magnitude limit within reach of the James Webb Space Telescope. Thus we predict that the best redshiftto search for luminous Pop III- bright galaxiesis just before reionization, while lensing surveys for fainter galaxiesshould push to the highest redshiftspossible.