Modeling Gamma‐Ray Attenuation in High Redshift GeV Spectra

We present two models for the cosmological UV background light, and calculate the opacity of GeV gamma—rays out to redshift 9. The contributors to the background include 2 possible quasar emissivities, and output from star—forming galaxies as determined by recent a semi—analytic model (SAM) of structure formation. The SAM used in this work is based upon a hierarchical build‐up of structure in a ΛCDM universe and is highly successful in reproducing a variety of observational parameters. Above 1 Rydberg energy, ionizing radiation is subject to reprocessing by the IGM, which we treat using our radiative transfer code, CUBA. The two models for quasar emissivity differing above z = 2.3 are chosen to match the ionization rates observed using flux decrement analysis and the higher values of the line‐of‐sight proximity effect. We also investigate the possibility of aflat star formation rate density at z>5. We conclude that observations of gamma—rays from 10 to 100 GeV by Fermi (GLAST) and the next generation of g...