He II optical depth and UV escape fraction of galaxies

TL;DR

This paper investigates how the escape of hydrogen-ionizing photons from high-redshift galaxies influences the He II ionizing ultraviolet background, affecting observations of the intergalactic medium and providing new constraints on galaxy evolution.

Contribution

It demonstrates that radiative transport effects of escaping H I ionizing photons can explain the observed increase in He II Lyman alpha optical depth around z~2.7, linking galaxy escape fractions to intergalactic medium properties.

Findings

Radiative transport of escaping photons modifies the He II UVB spectrum.

The increase in He II Lyman alpha optical depth at z~2.7 can be explained by these effects.

Escape fraction impacts fluctuations in the He II ionizing background.

Abstract

We study the effect of H I ionizing photons escaping from high-redshift (high-z) galaxies have on the He II ionizing ultraviolet background (UVB) radiation. While these photons do not directly interact with He II ions, we show that they play an important role, through radiative transport, in modifying the shape of He II ionizing part of UVB spectrum. Within the observed range of UV escape from galaxies, we show that the rapid increase in He II Lyman alpha effective optical depth at z~2.7 can naturally be explained by radiative transport effects. Therefore, the relationship between a well measured He II Lyman alpha effective optical depth and the redshift in the post-He II reionization era can be used to place additional constraints on the redshift evolution of UV escape from high-z galaxies. Our study also suggests that the escape fraction of H I ionizing photons from galaxies has an important role in the fluctuations of the He II ionizing UVB.