Extragalactic Background Light from Hierarchical Galaxy Formation:
  Gamma-ray Attenuation up to the Epoch of Cosmic Reionization and the First
  Stars
  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 and
following the cosmic reionization history in a simplified way, the model is
also broadly consistent with available data concerning reionization,
particularly the Thomson scattering optical depth constraints from WMAP. In
comparison with previous EBL studies up to z ~ 3-5, our predicted gamma-gamma
opacity is in general agreement for observed gamma-ray energy below 400/(1 + z)
GeV, whereas it is a factor of ~ 2 lower above this energy because of a
correspondingly lower cosmic star formation rate, even though the observed UV
luminosity is well reproduced by virtue of our improved treatment of dust
obscuration and direct estimation of star formation rate. The horizon energy at
which the gamma-ray opacity is unity does not evolve strongly beyond z ~ 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. The
detailed results of our model are publicly available in numerical form at the
URL http://www.slac.stanford.edu/~yinoue/Download.html.
