THE BIRTH OF A GALAXY: PRIMORDIAL METAL ENRICHMENT AND POPULATION II STELLAR POPULATIONS

Population III stars first form in dark matter halos with masses around 10 6 M� . By definition, they are metal-free, and their protostellar collapse is driven by molecular hydrogen cooling in the gas- phase, leading to a massive characteristic mass � 100 Mand suppressed fragmentation. Population II stars with lower characteristic masses form when the star-forming gas reaches a critical metallicity of 10 −6 10 −3:5 Z� , depending on whether dust cooling is important. We present adaptive mesh refinement radiation hydrodynamics simulations that follows the transition from Population III to II star formation. We model stellar radiative feedback with adaptive ray tracing. A top-heavy initial mass function for the Population III stars is considered, resulting in a plausible distribution of pair- instability supernovae and associated metal enrichment. We find that the gas fraction recovers from 5 percent to nearly the cosmic fraction in halos with merger histories rich in halos above 10 7 M� . A single pair-instability supernova is sufficient to enrich the host halo toa metallicity floor of 10 −3 Z� and to transition to Population II star formation. This provides a natural explanation for the observed floor on damped Lyman alpha (DLA) systems metallicities reported in the literature, which is of this order. We find that stellar metallicities do not necessarily trace stellar ages, as mergers of halos with established stellar populations can create superpositions of t Z evolutionary tracks. A bimodal metallicity distribution is created after a starburst occurs when the halo can cool efficiently through atomic line cooling. Subject headings: cosmology — methods: numerical — hydrodynamics — radiative transfer — star formation