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 the time-dependent and spatially resolved transfer of ionizing radiation in three dimensions, including the effects of dust absorption. For galaxies of total mass M ≳ 10 11 M ☉ and star formation rates SFR ≈ 1–5 M ☉ yr −1 , we find angular averaged escape fractions of 1%-3% 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 the H I disk. This fraction, and hence the escape fraction, is progressively smaller in disks of smaller galaxies because their H I disks are thicker and more extended relative to the distribution of young stars compared to massive galaxies. 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 sight to long-duration γ-ray bursts (GRBs). Using this method we find escape fractions of the GRB host galaxies of 2%-3%, consistent with our theoretical predictions. Our results thus suggest that high-redshift galaxies are inefficient in releasing ionizing radiation produced by young stars into the intergalactic medium.