CONSTRAINTS ON THE EXTRAGALACTIC BACKGROUND LIGHT FROM GAMMA-RAY OBSERVATIONS OF HIGH-REDSHIFT QUASARS

We propose to use the detectability of energetic gamma-rays in the 10- 200 GeV range from high-redshift quasars to set limits on the energy d ensity and era of formation of the extragalactic background light (EBL ) in the near-ultraviolet, optical, and near-infrared portion of the s pectrum. 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 a nd of the spectral energy distribution of young galaxies, and simple p arameterizations of the star formation history. We demonstrate that a cosmic background of optical photons which is comparable to the integr ated EBL contributed by ordinary galaxies and originates as near-ultra violet radiation at redshift z similar to 2 will make the universe opt ically thick to gamma-ray photons above similar to 30 GeV through elec tron-positron pair production. We also show that a detection by the EG RET instrument aboard the Compton Observatory of greater than or simil ar to 15 GeV photons from the quasar 1633 + 382 (Matter et al. 1993) w ould rule out models in which a diffuse optical background with an ene rgy density several times in excess that of known galaxies was formed at z similar to 2 by a new class of sources. The universe to intermedi ate redshifts is optically thin to pair production below z similar to 10 GeV.