Extragalactic background light and gamma-ray attenuation

Attenuation of high-energy gamma rays by pair production with UV, optical and IR background photons provides a link between the history of galaxy formation and high-energy astrophysics. I present new calculations of the EBL from a recent set of semi-analytic models (SAMs), based upon a ΛCDM hierarchical structural formation scenario and employing all ingredients thought to be important to galaxy formation and evolution, as well as reprocessing of starlight by dust to mid- and far-IR wavelengths. These models also use results from recent hydrodynamic galaxy merger simulations. These latest SAMs are successful in reproducing a large variety of observational constraints such as number counts, luminosity and mass functions, and color bimodality. We have created 2 models that bracket the likely ranges of galaxy emissivities, and for each of these we show how the optical depth from pair-production is affected by redshift and gamma-ray energy. Dust reemission in the IR is calculated using 2 different sets of templates. I discuss the implications of these models for blazar observations by current experiments, and compare with other models of the EBL that have been created using different techniques.;The second part of this work focuses specifically on the evolving UV background out to the epoch of cosmological reionization and makes predictions for the amount of GeV gamma-ray attenuation by electron-positron pair production. This calculation utilizes stellar emissivities from our SAMs along with estimates of quasar emission, and accounts 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. There are 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.;Gamma-ray bursts (GRBs) may be a source of high-redshift photons above 10 GeV, and could be useful as a probe of the evolving UV background radiation. In the last section of this work, we develop a simple phenomenological model for the number and redshift distribution of gamma-ray bursts that can be seen at GeV energies with the Fermi satellite and MAGIC atmospheric Cherenkov telescope. We calculate the number of gamma rays predicted per year, and show how this result is modified by considering interactions with different realizations of the evolving EBL. We also discuss bright Fermi GRB 080916C in the context of this model. The LAT on Fermi can be expected to see a small number of photons above 10 GeV each year from distant GRBs. Annual results for ground-based instruments like MAGIC are highly variable due to the low duty cycle and sky coverage of the telescope, however successfully viewing a bright or intermediate GRB could provide hundreds or even thousands of photons from high redshift, which would almost certainly be extremely useful in constraining the high-redshift EBL.