Absorption of High Energy Gamma Rays by Interactions With Starlight Photons in Extragalactic Space at High Redshifts and the High Energy Gamma-Ray Background

In this paper, we extend previous work on the absorption of high energy γ-rays in intergalactic space by calculating the absorption of 10 to 500 GeV γ-rays at high redshifts. This calculation requires the determination of the high-redshift evolution of the intergalactic starlight photon field, including its spectral energy distribution out to frequencies beyond the Lyman limit. To estimate this evolution, we have followed a recent analysis of Fall, Charlot & Pei, which reproduces the redshift dependence of the starlight background emissivity obtained by the Canada-France redshift survey group. We also include the UV background from quasars. We give our results for the γ-ray opacity as a function of redshift out to a redshift of 3. We also give predicted γ-ray spectra for selected blazars and extend our calculations of the extragalactic γ-ray background from blazars to an energy of 500 GeV with absorption effects included. Our results indicate that the extragalactic γ-ray background spectrum from blazars should steepen significantly above 20 GeV, owing to extragalactic absorption. Future observations of a such a steepening would thus provide a test of the blazar origin hypothesis for the γ-ray background radiation. We also note that our absorption calculations can be used to place limits on the redshifts of γ-ray bursts; for example, our calculated opacities indicate that the 17 Feb. 1994 burst observed by EGRET must have originated at z ≤∼ 2. Finally, our estimates of the high-energy γ-ray background spectrum are used to determine the observability of multi-GeV γ-ray lines from the annihilation of supersymmetric dark-matter particles in the galactic halo.