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The Effect of Galactic Properties on the Escape Fraction of Ionizing Photons

The escape fraction, fesc, of ionizing photons from early galaxies is a crucial parameter for determining whether the observed galaxies at z > 6 are able to reionize the high-redshift intergalactic medium. Previous attempts to measure fesc have found a wide range of values, varying from less than 0.01 to nearly 1. Rather than finding a single value of fesc, we clarify through modeling how internal properties of galaxies affect fesc through the density and distribution of neutral hydrogen within the galaxy, along with the rate of ionizing photons production. We find that the escape fraction depends sensitively on the covering factor of clumps, along with the density of the clumped and interclump medium. One must therefore be cautious when dealing with an inhomogeneous medium. Fewer, high-density clumps lead to a greater escape fraction than more numerous low-density clumps. When more ionizing photons are produced in a starburst, fesc increases, as photons escape more readily from the gas layers. Large variations in the predicted escape fraction, caused by differences in the hydrogen distribution, may explain the large observed differences in fesc among galaxies. Values of fesc must also be consistent with the reionization history. High-mass galaxies alone are unable to reionize the universe, because fesc > 1 would be required. Small galaxies are needed to achieve reionization, with greater mean escape fraction in the past.