ESCAPE FRACTION OF IONIZING PHOTONS DURING REIONIZATION: EFFECTS DUE TO SUPERNOVA FEEDBACK AND RUNAWAY OB STARS

Abstract

The fraction of hydrogen ionizing photons escaping from galaxies into the\nintergalactic medium is a critical ingredient in the theory of reionization. We\nuse two zoomed-in, high-resolution (4 pc), cosmological radiation hydrodynamic\nsimulations with adaptive mesh refinement to investigate the impact of two\nphysical mechanisms (supernova feedback and runaway OB stars) on the escape\nfraction (f_esc) at the epoch of reionization (z>7). We implement a new,\nphysically motivated supernova feedback model that can approximate the Sedov\nsolutions at all (from the free expansion to snowplow) stages. We find that\nthere is a significant time delay of about ten million years between the peak\nof star formation and that of escape fraction, due to the time required for the\nbuild-up and subsequent destruction of the star-forming cloud by supernova\nfeedback. Consequently, the photon number-weighted mean escape fraction for\ndwarf galaxies in halos of mass 10^8-10^10.5 Msun is found to be <fesc>~11%,\nalthough instantaneous values of f_esc>20% are common when star formation is\nstrongly modulated by the supernova explosions. We find that the inclusion of\nrunaway OB stars increases the mean escape fraction by 22% to <fesc>~14%. As\nsupernovae resulting from runaway OB stars tend to occur in less dense\nenvironments, the feedback effect is enhanced and star formation is further\nsuppressed in halos with Mvir>10^9 Msun in the simulation with runaway OB stars\ncompared with the model without them. While both our models produce enough\nionizing photons to maintain a fully ionized universe at z>7 as observed, a\nstill higher amount of ionizing photons at z>9 appears necessary to accommodate\nthe high observed electron optical depth inferred from cosmic microwave\nbackground observations.\n