The Escape Fraction of Ionizing Radiation from Galaxies

TL;DR

This paper presents semi-analytic calculations of the escape fractions of ionizing radiation from galaxies, highlighting the roles of galaxy properties, source spectra, and X-ray contributions, with implications for cosmic reionization.

Contribution

It introduces new calculations of escape fractions for helium ionizing radiation and examines the impact of X-rays on these escape fractions, advancing understanding of galaxy contributions to reionization.

Findings

X-rays significantly boost escape fractions for hydrogen and helium

Helium ionization fronts can lead or follow hydrogen fronts depending on source hardness

Escape fractions vary with galaxy type, source location, and gas distribution

Abstract

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.