Origin Of Extragalactic Gamma-ray Background Research Articles

Contribution from star-forming galaxies to the cosmic gamma-ray background radiation

The origin of extragalactic gamma-ray background (EGRB) has been discussed for a long time and various sources have been discussed as possible contributors to EGRB, such as active galactic nuclei, star-forming galaxies, galaxy clusters, structure formation, and dark matter annihilation. We present a new theoretical calculation of the contribution to the EGRB 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 galaxies as well as high redshifts. Gamma-ray luminosity and spectrum of galaxies are related to star formation rate, gas mass, and star formation mode (quiescent or starburst) of model galaxies using latest observed data of nearby galaxies. Our standard model predicts ∼ 14% contribution from star-forming galaxies to the total EGRB flux recently reported by the Fermi Gamma-Ray Space Telescope. The predicted number of nearby galaxies detectable by Fermi is consistent with the observation. The sum of the known contributions from AGNs and star-forming galaxies can explain a large part of EGRB, with a remarkable agreement between the predicted model spectrum and observation.

Extragalactic Gamma-ray Background Radiation from Beamed and Unbeamed Active Galactic Nuclei

The origin of the extragalactic gamma-ray background (EGRB) radiation has been a mystery in astrophysics for a long time. Recently the Fermi gamma-ray satellite (Fermi) has revealed that ∼22% of the unresolved EGRB would be explained by blazars, which are one population of beamed active galactic nuclei (AGNs). The remaining ∼78% of the unresolved EGRB is still unknown. We estimate the contribution of gamma-ray loud radio galaxies, which are misaligned radio loud AGNs recently detected by Fermi, to EGRB using the radio luminosity function of radio-loud AGNs with the correlation between the radio and gamma-ray luminosities. We find that ∼25% of the unresolved EGRB will be explained by gamma-ray loud radio galaxy population. We also discuss further about the origin of EGRB by comparing the Fermi EGRB data with our studies on various AGN populations' contribution to EGRB, which are radio quiet AGNs, blazars, and gamma-ray loud radio galaxies.