Contribution of gamma-ray-loud radio galaxies' core emissions to the cosmic MeV and GeV gamma-ray background radiation

Abstract

The Fermi gamma-ray satellite has recently detected gamma-ray emissions from radio galaxy cores. From these samples, we first examine the correlation between the luminosities at 5 GHz, L 5 GHz, and at 0.1-10 GeV, L γ, of gamma-ray-loud radio galaxies. We find that the correlation is significant with L γ ∝ L1.165 GHz based on a partial correlation analysis. Using this correlation and the radio luminosity function (RLF) of radio galaxies, we explore the contribution of gamma-ray-loud radio galaxies to the unresolved extragalactic gamma-ray background (EGRB). The gamma-ray luminosity function is obtained by normalizing the RLF to reproduce the source-count distribution of the Fermi gamma-ray-loud radio galaxies. We find that gamma-ray-loud radio galaxies can explain ∼25% of the unresolved Fermi EGRB flux above 100MeV and will also make a significant contribution to the EGRB in the 1-30MeV energy band. Since blazars explain 22% of the EGRB above 100MeV, radio-loud active galactic nucleus populations explain ∼47% of the unresolved EGRB. We further make an interpretation on the origin of the EGRB. The observed EGRB spectrum at 0.2-100 GeV does not show an absorption signature by the extragalactic background light. Thus, the dominant population of the origin of EGRB at very high energy (>30 GeV) might be either nearby gamma-ray-emitting sources or sources with very hard gamma-ray spectra. © 2011. The American Astronomical Society. All rights reserved..