External Enrichment of Minihalos by the First Supernovae

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 Mpc³ 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 50% of metal-enriched halos is supernova explosions of Population III stars occurring 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 10⁶ M_⊙ down to z = 9.3. Despite the prevalence of external enrichment in low-mass halos, Population 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 toward 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_⊙. We also find that the fraction of externally enriched halos increases with time: ~90% of halos that are externally enriched have M_vir < 10⁶ M_⊙, and that pair-instability supernovae contribute the most to the enrichment of the intergalactic medium as a whole and are thus are the predominant supernova type contributing to the external enrichment of halos.