ABSORPTION OF HIGH-ENERGY GAMMA-RAYS BY INTERACTIONS WITH EXTRAGALACTIC STARLIGHT PHOTONS AT HIGH REDSHIFTS AND THE HIGH-ENERGY GAMMA-RAY BACKGROUND

In this paper, we extend previous work on the absorption of high-energ y gamma-rays in intergalactic space by calculating the absorption of 1 0-500 GeV gamma-rays at high redshifts. This calculation requires the determination of the high-redshift evolution of the intergalactic star light photon field, including its spectral energy distribution out to frequencies beyond the Lyman limit. To estimate this evolution, we hav e followed a recent analysis by Fall, Chariot, & Pei, which reproduces the redshift dependence of the star-light background emissivity obtai ned by the Canada-France Redshift Survey group. We give our results fo r the gamma-ray opacity as a function of redshift out to a redshift of z = 3. We also give predicted gamma-ray spectra for selected blazars and extend our calculations of the extragalactic gamma-ray background from blazars to an energy of 500 GeV with absorption effects included. Our results indicate that the extragalactic gamma-ray background spec trum from blazars should steepen significantly above 20 GeV, owing to extragalactic absorption. Future observations of a such a steepening w ould thus provide a test of the blazar origin hypothesis for the gamma -ray background radiation. We also note that our absorption calculatio ns can be used to place limits on the redshifts of gamma-ray bursts; f or example, our calculated opacities indicate that the 1994 February 1 7 burst observed by EGRET most probably originated at z less than or e qual to similar to 2.