Extragalactic Background Light from Hierarchical Galaxy Formation: Gamma-ray Attenuation up to the Epoch of Cosmic Reionization and the First Stars

TL;DR

This paper introduces a new semi-analytical model of the extragalactic background light (EBL) based on hierarchical galaxy formation, accounting for Population III stars and cosmic reionization, to predict gamma-ray attenuation up to redshift 10.

Contribution

The model uniquely combines galaxy formation, reionization history, and star contributions, providing updated gamma-ray opacity predictions consistent with observations up to z=10.

Findings

Gamma-ray opacity below 400/(1+z) GeV agrees with previous studies.

Opacity is about twice lower above this energy due to lower star formation rate.

Population III stars contribute minimally to the EBL at z=0 and are hard to detect via gamma-ray absorption.

Abstract

We present a new model of the extragalactic background light (EBL) and corresponding gamma-gamma opacity for intergalactic gamma-ray absorption from z = 0 up to z = 10, based on a semi-analytical model of hierarchical galaxy formation that reproduces key observed properties of galaxies at various redshifts. Including the potential contribution from Population III stars and following the cosmic reionization history in a simplified way, the model is also broadly consistent with available data concerning reionization, particularly the Thomson scattering optical depth constraints from WMAP. In comparison with previous EBL studies up to z ~ 3-5, our predicted gamma-gamma opacity is in general agreement for observed gamma-ray energy below 400/(1 + z) GeV, whereas it is a factor of ~ 2 lower above this energy because of a correspondingly lower cosmic star formation rate, even though the observed UV luminosity is well reproduced by virtue of our improved treatment of dust obscuration and direct estimation of star formation rate. The horizon energy at which the gamma-ray opacity is unity does not evolve strongly beyond z ~ 4 and approaches ~ 20 GeV. The contribution of Population III stars is a minor fraction of the EBL at z = 0, and is also difficult to distinguish through gamma-ray absorption in high-z objects, even at the highest levels allowed by the WMAP constraints. Nevertheless, the attenuation due to Population II stars should be observable in high-z gamma-ray sources by telescopes such as Fermi or CTA and provide a valuable probe of the evolving EBL in the rest-frame UV. The detailed results of our model are publicly available in numerical form at the URL http://www.slac.stanford.edu/~yinoue/Download.html.