Fully Coupled Simulation of Cosmic Reionization. III. Stochastic Early
  Reionization by the Smallest Galaxies
  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 $M_h =10^{6.5}-10^{8} M_{\odot}$ that
have been chemically enriched by supernova ejecta from prior Pop 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, due 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 inline 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 $10^8 M_{\odot}$ 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 $\sim 10^9 M_{\odot}$
and above begin to dominate the ionizing emissivity, does reionization proceed
smoothly in the usual manner deduced from previous studies. This occurs at
$z\approx 10$ in our simulation.
