Reconstructing the γ-ray photon optical depth of the universe to z 4 from multiwavelength galaxy survey data

Abstract

We reconstruct the γ-ray opacity of the universe out to z ≲ 3-4 using an extensive library of 342 observed galaxy luminosity function (LF) surveys extending to high redshifts. We cover the whole range from UV to mid-IR (0.15-25 μm) providing for the first time a robust empirical calculation of the γγ optical depth out to several TeV. Here, we use the same database as Helgason etal. where the extragalactic background light was reconstructed from LFs out to 4.5 μm and was shown to recover observed galaxy counts to high accuracy. We extend our earlier library of LFs to 25 μm such that it covers the energy range of pair production with γ-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 γ-rays at all Fermi/LAT energies out to z 2 whereas it becomes opaque to TeV photons already at z ≲ 0.2 and reaching τ 10 at z = 1. Comparing with the currently available Fermi/LAT gamma-ray burst and blazar data shows that there is room for significant emissions originating in the first stars era. © 2012. The American Astronomical Society. All rights reserved.