Fully Coupled Simulation of Cosmic Reionization. III. Stochastic Early Reionization by the Smallest Galaxies

Abstract

Abstract Previously we identified a new class of early galaxy that we estimate contributes up to 30% of the ionizing photons responsible for reionization. These are low-mass halos in the range = 106.5–108 that have been chemically enriched by supernova ejecta from prior Population III star formation. Despite their low star formation rates, these metal cooling halos (MCs) are significant sources of ionizing radiation, especially at the onset of reionization, owing to their high number density and ionizing escape fractions. Here we present a fully coupled radiation hydrodynamic simulation of reionization that includes these MCs, as well the more massive hydrogen atomic line cooling halos. Our method is novel: we perform halo finding in line with the radiation hydrodynamical simulation and assign escaping ionizing fluxes to halos using a probability distribution function (pdf) measured from the galaxy-resolving Renaissance Simulations. The pdf captures the mass dependence of the ionizing escape fraction, as well as the probability that a halo is actively forming stars. With MCs, reionization starts earlier than if only halos of 108 and above are included; however, the redshift when reionization completes is only marginally affected, as this is driven by more massive galaxies. Because star formation is intermittent in MCs, the earliest phase of reionization exhibits a stochastic nature, with small H ii regions forming and recombining. Only later, once halos of mass ∼109 and above begin to dominate the ionizing emissivity, does reionization proceed smoothly in the usual manner deduced from previous studies. This occurs at z ≈ 10 in our simulation.