Detailed Modelling Of The Ebl Along Vhe Gamma-Ray Paths

Interactions between the extragalactic background light (EBL) and very high-energy (VIIE; E > 10 GeV) gamma-rays from cosmological sources alter their gamma-ray spectrum. The stronger absorption of harder gamma-rays causes a steepening of the observed gamma-ray spectrum at the highenergy tail, which can be expressed by an increase of the power index. The effect provides a link between high-energy astrophysics and the evolution of galaxies, In this work we develop a new hybrid EBL model by augmenting our previous analytic model with information from semi-analytic galaxy catalogues. The model allows us to study the gamma-ray opacities of individual gamma-ray paths through the (simulated) universe and evaluate the effect of local fluctuations in the EDI, intensity along each path. We confirm an order of magnitude fluctuations in the local EBL (based on the cumulative light from model galaxy distributions within spheres of 50 h(-1) Mpc). However, the effect of these fluctuations on the VHF gamma-ray opacity is insignificant due to the overall very small contribution of the local EBL to the total EBL. We also investigate the effect that a galaxy crossing the line of sight of the gamma-ray source may have on the VHE spectrum. We find that galaxies with stellar masses of M > 10(11) M-circle dot could have a significant effect on the gamma-ray absorption. It is unlikely that the observed variation in the spectral index is caused by the proximity of single galaxies to the gamma-ray paths as these are extremely rare.