Following the Cosmic Evolution of Pristine Gas. III. The Observational Consequences of the Unknown Properties of Population III Stars

We study the observational consequences of several unknown properties of Population III stars using large-scale cosmological simulations that include a subgrid model to track the unresolved mixing of pollutants. Varying the value of the critical metallicity that marks the boundary between Population III and Population II star formation across 2 dex has a negligible effect on the fraction of Population III stars formed and the subsequent fraction of Population III flux from high-redshift galaxies. However, adopting a lognormal initial mass function (IMF) for Population III stars, in place of a baseline Salpeter IMF, results in a Population III star formation rate density that is 1/4 of the baseline rate. The flux from high-redshift galaxies modeled with this IMF is highly bimodal, resulting in a tiny fraction of z ≤ 8 galaxies with more than 75% of their flux coming from Population III stars. However, at z = 9, right before reionization in our simulations, ≈20% of galaxies are Population III-bright with {m}_UV}≤slant 31.4 mag, and at least 75% of their flux is generated by Population III stars. Additionally, the lognormal Population III IMF results in a population of carbon-enhanced, metal-poor stars in reasonable agreement with MW halo observations. Our analysis supports the conclusion that the Population III IMF was dominated by stars in the 20-120 {M}_⊙ range that generate supernovae with carbon-enhanced ejecta.