Escape of Ionizing Radiation from High-Redshift Galaxies

We use a three-dimensional radiation transfer code to calculate the steady state escape fraction of ionizing photons from disk galaxies as a function of redshift and galaxy mass. The gaseous disks are assumed to be isothermal (with a sound speed c(s) similar to 10 km s(-1)) and radially exponential. Their scale radius is related to the characteristic spin parameter and virial radius of their host halos, and their vertical structure is dictated by their self-gravity. The sources of radiation are taken to be either stars embedded in the disk or a central quasar. The predicted increase in the disk density with redshift results in an overall decline of the escape fraction with increasing redshift. For typical parameters of smooth disks, we find that the escape fraction at z similar to 10 is less than or similar to1% for stars but greater than or similar to 30% for miniquasars. Unless the smooth gas content of high-redshift disks was depleted by more than an order of magnitude due to supernovae-driven outflows or fragmentation, the reionization of the universe was most likely dominated by miniquasars rather than by stars.