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The fraction of hydrogen ionizing photons escaping from galaxies into the intergalactic medium is a critical ingredient in the theory of reionization. We use two zoomed-in, high-resolution (4 pc), cosmological radiation hydrodynamic simulations with adaptive mesh refinement to investigate the impact of two physical mechanisms (supernova feedback and runaway OB stars) on the escape fraction (fₑsc) at the epoch of reionization (z>7). We implement a new, physically motivated supernova feedback model that can approximate the Sedov solutions at all (from the free expansion to snowplow) stages. We find that there is a significant time delay of about ten million years between the peak of star formation and that of escape fraction, due to the time required for the build-up and subsequent destruction of the star-forming cloud by supernova feedback. Consequently, the photon number-weighted mean escape fraction for dwarf galaxies in halos of mass 10⁸-10¹0. 5 Msun is found to be ~11%, although instantaneous values of fₑsc>20% are common when star formation is strongly modulated by the supernova explosions. We find that the inclusion of runaway OB stars increases the mean escape fraction by 22% to ~14%. As supernovae resulting from runaway OB stars tend to occur in less dense environments, the feedback effect is enhanced and star formation is further suppressed in halos with Mvir>10⁹ Msun in the simulation with runaway OB stars compared with the model without them. While both our models produce enough ionizing photons to maintain a fully ionized universe at z>7 as observed, a still higher amount of ionizing photons at z>9 appears necessary to accommodate the high observed electron optical depth inferred from cosmic microwave background observations.
Kimm et al. (Thu,) studied this question.