The Extragalactic Optical-infrared Background Radiations, Their Time Evolution and the Cosmic Photon-photon Opacity

The background radiations in the optical and the infrared constitute a relevant cause of energy loss in the propagation of high energy particles through space. In particular, TeV observations with Cherenkov telescopes of extragalactic sources are influenced by the opacity effects due to the interaction of the very high-energy source photons with the background light. With the aim of assessing with the best possible detail these opacity terms, we have modelled the extragalactic optical and infrared backgounds using available information on cosmic sources in the universe from far-UV to sub-millimeter wavelengths over a wide range of cosmic epochs. We have exploited the relevant cosmological survey data – including number counts, redshift distributions, luminosity functions – from ground-based observatories in the optical, near-IR, and sub-millimeter, as well as multi-wavelength information coming from space telescopes, HST, ISO and Spitzer. Additional constraints have been used from direct measurements or upper limits on the extragalactic backgrounds by dedicated missions (COBE). All data were fitted and interpolated with a multi-wavelength backward evolutionary model, allowing us to estimate the background photon density and its redshift evolution. From the redshift-dependent background spectrum, the photon-photon opacities for sources of high-energy emission at any redshifts were then computed. The same results can also be used to compute the optical depths for any kind of processes in the intergalactic space involving interactions with background photons (like scattering of cosmicray particles). We have applied our photon-photon opacity estimates to the analysis of spectral data at TeV energies on a few BLAZARs of particular interest. The opacity-corrected TeV spectra are entirely consistent with standard photon-generation processes and show photon indices in any case steeper than Γintrinsic = 1.6. Contrary to some previous claims, but quite in agreement with Aharonian et al. (2006), we find no evidence for any truly diffuse background components in addition to those from resolved sources. We have tested in particular the effects of a photon background originating at very high redshifts, like would be the emissions by a primeval population of Population III stars around z ∼ 10. We could not identify any opacity features in our studied BLAZAR spectra consistent with such an emission and put a stringent limit on such diffuse photon intensity of ∼ 6 nW/m2/sr between 1 and 4 μm. TeV observations of BLAZARs are consistent with background radiations contributed by resolved galaxies in the optical and IR, and exclude prominent additional components from very high-z unresolved sources.