GeV Gamma-Ray Attenuation and the High-Redshift UV Background
  We present new calculations of the evolving UV background out to the epoch of
cosmological reionization and make predictions for the amount of GeV gamma-ray
attenuation by electron-positron pair production. Our results are based on
recent semi-analytic models of galaxy formation, which provide predictions of
the dust-extinguished UV radiation field due to starlight, and empirical
estimates of the contribution due to quasars. We account for the reprocessing
of ionizing photons by the intergalactic medium. We test whether our models can
reproduce estimates of the ionizing background at high redshift from flux
decrement analysis and proximity effect measurements from quasar spectra, and
identify a range of models that can satisfy these constraints. Pair-production
against soft diffuse photons leads to a spectral cutoff feature for gamma rays
observed between 10 and 100 GeV. This cutoff varies with redshift and the
assumed star formation and quasar evolution models. We find only negligible
amounts of absorption for gamma rays observed below 10 GeV for any emission
redshift. With observations of high-redshift sources in sufficient numbers by
the Fermi Gamma-ray Space Telescope and new ground-based instruments it should
be possible to constrain the extragalactic background light in the UV and
optical portion of the spectrum.
