Constraints on the Extragalactic Background Light from Gamma-Ray Observations of High-Redshift Quasars

We propose to use the detectability of energetic γ-rays in the 10-200 GeV range from high-redshift quasars to set limits on the energy density and era of formation of the extragalactic background light (EBL) in the near-ultraviolet, optical, and near-infrared portion of the spectrum. We study a variety of EBL models based on recent estimates of the density of starlight at the present epoch, a detailed modeling of the transfer of ionizing radiation through the intergalactic medium and of the spectral energy distribution of young galaxies, and simple parameterizations of the star formation history. We demonstrate that a cosmic background of optical photons which is comparable to the integrated EBL contributed by ordinary galaxies and originates as near-ultraviolet radiation at redshift z ∼ 2 will make the universe optically thick to γ-ray photons above γ30 GeV through electron-positron pair production. We also show that a detection by the EGRET instrument aboard the Compton Observatory of 15 GeV photons from the quasar 1633 + 382 (Mattox et al. 1993) would rule out models in which a diffuse optical background with an energy density several times in excess that of known galaxies was formed at z ∼ 2 by a new class of sources. The universe to intermediate redshifts is optically thin to pair production below ∼ 10 GeV.