The First Galaxies and the Effect of Heterogeneous Enrichment from Primordial Stars

Abstract

We incorporate new scale-intelligent models of metal-enriched star formation (\starss) with surrogate models of primordial stellar feedback (\starnet) into the astrophysics simulation code \enzo to analyze the impact of heterogeneous metal enrichment on the first galaxies. Our study includes the earliest generations of stars and the protogalaxies ($10^6 \lesssim M_v/M_\odot \lesssim 10^8$) containing them. We compare results obtained with the new methods to two common paradigms of metallicity initial conditions in simulations: ignoring the metallicity initial condition and assuming a uniform metallicity floor. We find that ignoring metallicity requirements for enriched star formation results in a redshift-dependent excess in stellar mass created and compounding errors consisting of stars forming in pristine gas. We find that using a metallicity floor causes an early underproduction of stars before $z=21$ that reverses to overproduction by $z=18$. At the final redshift, $z=14.95$, there is $\sim 20\%$ excess stellar mass with 8.6\% increased protogalaxy count. Heterogeneous metallicity initial conditions greatly increase the range of halo observables, e.g., stellar metallicity, stellar mass, and luminosity. The increased range leads to better agreement with observations of ultra-faint dwarf galaxies when compared to metallicity-floor simulations. \starnet generates protogalaxies with low stellar mass, $M_* \lesssim 10^3 M_\odot$, so may also serve to model low-luminosity protogalaxies more effectively than a metallicity floor criterion at similar spatial and mass resolution.