Direct Numerical Simulation of Reionization II: Recombinations, Clumping Factors, and the Photon Budget for Reionization

Abstract: In this first of several application papers, we investigate the mechanics of reionization from stellar sources in high-z galaxies, the utility of various clumping factors on estimating the recombination time in the IGM, and the photon budget required to achieve reionization. We test the accuracy of the static and time-dependent models of Madau et al. as predictors of reionization completion/maintenance. We simulate a WMAP7 LCDM cosmological model in a 20 Mpc comoving cube with 800^3 uniform fluid cells and dark matter particles. By tuning our star formation to approximately match the observed star formation rate density and luminosity function, we created a fully coupled radiation-hydro realization of H reionization which begins to ionize at z~10 and completes at z~5.8. We find that roughly 2 ionizing photons per H atom are required to convert the neutral IGM to a highly ionized state, which supports the "photon starved" scenario discussed by Bolton & Haehnelt. The events during reionization that lead to this number can generally be described as inside-out, but in reality the narrative depends on the level of ionization of the gas one attributes to as ionized. We find that the formula for the UV photon production rate dN/dt_ion(z) needed to maintain the IGM in an ionized state derived by Madau et al. should not be used to predict the epoch of reionization completion because it ignores history-dependent terms in the global ionization balance which are not ignorable. We find that the time-dependent model for the ionized volume fraction Q_HII is more predictive, but overestimates the redshift of reionization completion by delta_z~1. We propose a revised formulation of the time-dependent model which agrees with our simulation to O(1%). Finally, we use our simulation to estimate a global UV escape fraction due to circumgalactic gas resolved on our mesh to be <f_esc>~0.7.