Radiative Transfer Simulations of Cosmic Reionization With Pop II and III Stars

We have simulated 3 large volume, high resolution realizations of cosmic reionization using a hybrid code that combines a N‐body algorithm for dark matter, prescriptions for baryons and star formation, and a radiative transfer algorithm for ionizing photons. Our largest simulation, with 24 billion particles in a $100 Mpc/h$ box, simultaneously provides (1) the mass resolution needed to resolve dark matter halos down to a virial temperatures of $104 K$ and (2) the volume needed to fairly sample highly biased sources and large HII regions. We model the stellar initial mass function (IMF) by following the spatially dependent gas metallicity evolution, and distinguish between the first generation (Population III) stars and the second generation (Population II) stars. The Population III stars, with a top‐heavy IMF, produce an order of magnitude more ionizing photons at high redshifts $z≳10,$ resulting in a more extended reionization. In our simulations, complete overlap of HII regions occurs at $z∼6.5$ and the computed mass and volume weighted residual HI fractions at $5≲z≲6.5$ are both in good agreement with high redshift quasar absorption measurements from SDSS. The values for the Thomson optical depth are consistent within $1−σ$ of the current best‐fit value from the WMAP Year 3 data release.