Extragalactic Background Light up to the Epoch of Cosmic Reionization

We present a new model of the extragalactic background light (EBL) and corresponding gamma-gamma opacity for intergalactic gamma-ray absorption from z=0 up to z=10, based on a semi-analytical model of hierarchical galaxy formation that reproduces key observed properties of galaxies at various redshifts. Including the potential contribution from Population III stars in a simplified way, the model is also broadly consistent with available data concerning cosmic reionization, particularly the Thomson scattering optical depth constraints from WMAP. The horizon energy at which the gamma-ray opacity is unity does not evolve strongly beyond 4 and approaches ~20 GeV. The contribution of Population III stars is a minor fraction of the EBL at z=0, and is also difficult to distinguish through gamma-ray absorption in high-z objects, even at the highest levels allowed by the WMAP constraints. Nevertheless, the attenuation due to Population II stars should be observable in high-z gamma-ray sources by telescopes such as Fermi or CTA and provide a valuable probe of the evolving EBL in the rest-frame UV.