The Importance of the Population III Initial Mass Function in Determining the Characteristics of the Earliest Galaxies

Abstract: We use large-scale cosmological simulations to study the prospect of observing Population III (Pop III) bright galaxies with the James Webb Space Telescope (JWST). To quantify the impact of radiative transfer (RT), we compare a simulation that includes moment-based RT with one in which RT is handled approximately. Both simulations include a subgrid model of turbulent mixing, which is essential in tracking the formation of Pop III stars. We find that RT has a minor impact on our results and that the overall star formation rate densities for both simulations are in fair agreement with observations and other simulations. While our overall galaxy luminosity functions are consistent with current high-redshift observations, we predict a drop of a factor of at least 6 in detectable galaxy counts at $z = 14$ as compared to $z = 12$ at $M_{ab}$ $\le$ -16. Modeling Pop III stars according to a lognormal, top-heavy initial mass function (IMF), we find that these stars contribute no more than about 1 percent of the flux of potentially detectable lensed galaxies at $z = 12$-14 with $M_{ab} \le -15$. This is because a top-heavy Pop III IMF results in 99 percent of Pop III stellar mass being recycled within 10 Myr, well before the approx. 30 Myr timescale on which galaxies recover from supernova feedback and heating. These effects conspire to quickly extinguish Pop III star formation, making their detection difficult even for JWST.