The Escape Fraction of Ionizing Radiation from Galaxies
  The escape of ionizing radiation from galaxies plays a critical role in the
evolution of gas in galaxies, and the heating and ionization history of the
intergalactic medium. We present semi-analytic calculations of the escape
fraction of ionizing radiation for both hydrogen and helium from galaxies
ranging from primordial systems to disk-type galaxies that are not heavily
dust-obscured. We consider variations in the galaxy density profile, source
type, location, and spectrum, and gas overdensity/distribution factors. For
sufficiently hard first-light sources, the helium ionization fronts closely
track or advance beyond that of hydrogen. Key new results in this work include
calculations of the escape fractions for He I and He II ionizing radiation, and
the impact of partial ionization from X-rays from early AGN or stellar clusters
on the escape fractions from galaxy halos. When factoring in
frequency-dependent effects, we find that X-rays play an important role in
boosting the escape fractions for both hydrogen and helium, but especially for
He II. We briefly discuss the implications of these results for recent
observations of the He II reionization epoch at low redshifts, as well as the
UV data and emission-line signatures from early galaxies anticipated from
future satellite missions.
