CONTRIBUTION FROM STAR-FORMING GALAXIES TO THE COSMIC GAMMA-RAY BACKGROUND RADIATION

We present a new theoretical calculation of the contribution to the extragalactic gamma-ray background (EGRB) radiation from star-forming galaxies, based on a state-of-the-art model of hierarchical galaxy formation that is in quantitative agreement with a variety of observations of local and high-redshift galaxies. Gamma-ray luminosity (L-gamma) and the spectrum of galaxies are related to the star formation rate (psi), gas mass (M-gas), and star formation mode (quiescent or starburst) of model galaxies using the latest observed data of nearby galaxies. We try the two limiting cases about gamma-ray production: the escape limit (L-gamma proportional to psi M-gas) and the calorimetric limit (L-gamma proportional to psi), and our standard model predicts 7% and 4% contribution from star-forming galaxies to the total EGRB flux (including bright resolved sources) recently reported by the Fermi Gamma-Ray Space Telescope. Systematic uncertainties do not allow us to determine the EGRB flux better than by a factor of similar to 2. The predicted number of nearby galaxies detectable by Fermi is consistent with the observation. Intergalactic absorption by pair-production attenuates the EGRB flux only by a modest factor of similar to 1.3 at the highest Fermi energy band, and the reprocessed cascade emission does not significantly alter EGRB at lower photon energies. The sum of the known contributions from active galactic nuclei and star-forming galaxies can explain a large part of EGRB, with a remarkable agreement between the predicted model spectrum and observation.