Reconstructing the \gamma-ray Photon Optical Depth of the Universe to z~4 from Multiwavelength Galaxy Survey Data

TL;DR

This paper empirically reconstructs the gamma-ray optical depth of the universe up to redshift 4 using extensive galaxy survey data, providing new insights into gamma-ray transparency and the contribution of early stars.

Contribution

It extends previous luminosity function libraries to mid-infrared wavelengths, enabling a robust empirical calculation of gamma-ray optical depth across a wide energy range and redshifts.

Findings

The universe is largely transparent to gamma-rays up to redshift 2 at Fermi energies.

It becomes opaque to TeV photons at redshift below 0.2.

There is potential for significant gamma-ray emission from the first stars era.

Abstract

We reconstruct \gamma-ray opacity of the Universe out to z<3-4 using an extensive library of 342 observed galaxy luminosity function surveys extending to high redshifts. We cover the whole range from UV to mid-IR (0.15-25mic) providing for the first time a robust empirical calculation of the \gamma\gamma-optical depth out to several TeV. Here, we use the same database as Helgason et al. 2012 where the EBL was reconstructed from luminosity functions out to 4.5mic and was shown to recover observed galaxy counts to high accuracy. We extend our earlier library of LFs to 25mic such that it covers the energy range of pair production with \gamma-rays 1) in the entire Fermi/LAT energy range, and 2) at higher TeV energies probed by ground-based Cherenkov telescopes. In the absence of significant contributions to the cosmic diffuse background from unknown populations, such as the putative Population III era sources, the Universe appears to be largely transparent to \gamma-rays at all Fermi/LAT energies out to z~2 whereas becoming opaque to TeV photons already at z<0.2 and reaching \tau~10 at z=1. Comparing with the currently available Fermi/LAT GRB and blazar data shows that there is room for significant emissions originating in the first stars era.