Escape of Ionizing Radiation from High Redshift Galaxies

Abstract

We model the escape of ionizing radiation from high-redshift galaxies using high-resolution Adaptive Mesh Refinement N-body + hydrodynamics simulations. Our simulations include time-dependent and spatially-resolved transfer of ionizing radiation in three dimensions, including effects of dust absorption. For galaxies of total mass M > 10^11 Msun and star formation rates SFR ~ 1-5 Msun/yr, we find angular averaged escape fractions of 0.01-0.03 over the entire redshift interval studied (3<z<9). In addition, we find that the escape fraction varies by more than an order of magnitude along different lines-of-sight within individual galaxies, from the largest values near galactic poles to the smallest along the galactic disk. The escape fraction declines steeply at lower masses and SFR. We show that the low values of escape fractions are due to a small fraction of young stars located just outside the edge of HI disk. We compare our predicted escape fraction of ionizing photons with previous results, and find a general agreement with both other simulation results and available direct detection measurements at z ~ 3. We also compare our simulations with a novel method to estimate the escape fraction in galaxies from the observed distribution of neutral hydrogen column densities along the lines of sights to long duration gamma-ray bursts. Using this method we find escape fractions of the GRB host galaxies of 2-3%, consistent with our theoretical predictions. [abridged]