GeV Gamma-Ray Attenuation and the High-Redshift UV Background

TL;DR

This paper models the evolving UV background and predicts GeV gamma-ray attenuation due to pair production, providing insights into the high-redshift universe and the extragalactic background light.

Contribution

It offers new semi-analytic models of the UV background and predicts gamma-ray attenuation features, linking galaxy formation, quasars, and gamma-ray observations.

Findings

Gamma-ray spectral cutoff varies with redshift and models.

Negligible absorption below 10 GeV for all redshifts.

Models can match high-redshift ionizing background estimates.

Abstract

We present new calculations of the evolving UV background out to the epoch of cosmological reionization and make predictions for the amount of GeV gamma-ray attenuation by electron-positron pair production. Our results are based on recent semi-analytic models of galaxy formation, which provide predictions of the dust-extinguished UV radiation field due to starlight, and empirical estimates of the contribution due to quasars. We account for the reprocessing of ionizing photons by the intergalactic medium. We test whether our models can reproduce estimates of the ionizing background at high redshift from flux decrement analysis and proximity effect measurements from quasar spectra, and identify a range of models that can satisfy these constraints. Pair-production against soft diffuse photons leads to a spectral cutoff feature for gamma rays observed between 10 and 100 GeV. This cutoff varies with redshift and the assumed star formation and quasar evolution models. We find only negligible amounts of absorption for gamma rays observed below 10 GeV for any emission redshift. With observations of high-redshift sources in sufficient numbers by the Fermi Gamma-ray Space Telescope and new ground-based instruments it should be possible to constrain the extragalactic background light in the UV and optical portion of the spectrum.