External Enrichment of Minihalos by the First Supernovae

Abstract

Recent high-resolution simulations of early structure formation have shown that externally enriched halos may form some of the first metal enriched stars. This study utilizes a 1 comoving Mpc$^3$ high-resolution simulation to study the enrichment process of metal-enriched halos down to $z=9.3$. Our simulation uniquely tracks the metals ejected from Population III stars, and we use this information to identify the origin of metals within metal-enriched halos. These halos show a wide range of metallicities, but we find that the source of metals for $\gtrsim$ 50\% of metal-enriched halos is supernova explosions of Population III stars occuring outside their virial radii. The results presented here indicate that external enrichment by metal-free stars dominates the enrichment process of halos with virial mass below $10^{6}\,M_\odot$ down to $z=9.3$. Despite the prevalence of external enrichment in low mass halos, Pop II stars forming due to external enrichment are rare because of the small contribution of low-mass halos to the global star formation rate combined with low metallicities towards the center of these halos resulting from metal ejecta from external sources mixing from the outside-in. The enriched stars that do form through this process have absolute metallicities below $10^{-3}\,Z_\odot$. We also find that the fraction of externally enriched halos increases with time, $\sim 90\%$ of halos that are externally enriched have $M_\mathrm{vir} < 10^6\,M_\odot$, and that pair-instability supernovae contribute the most to the enrichment of the IGM as a whole and are thus are the predominant supernova type contributing to the external enrichment of halos.