The Gamma-Ray Emission from Radio Galaxies and Their Contribution to the Isotropic Gamma-Ray Background

We evaluate the contribution to the isotropic gamma-ray background (IGRB) coming from radio galaxies (RGs), the subclass of radio-loud active galactic nuclei with the highest misalignment from the line of sight. Since only a small number of RGs are detected in gamma-rays compared to the largest known radio population, the correlation between radio and gamma-ray emission serves a crucial tool to characterize the gamma-ray properties of these sources. We analyze the population of RGs using two samples. The first sample contains 26 sources individually detected by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope at gamma-rays. The second sample contains 210 RGs for which the gamma-ray emission is not significantly detected by the LAT. We use a stacking analysis to characterize the average properties of the gamma-ray emission of the two samples, separately at first and then combined. We then evaluate the correlation between their gamma-ray emission and the emission from their radio core at 5 GHz, and we use it to determine their contribution to the IGRB. Due to the limited number of RGs detected at the gamma-rays, information on the gamma-ray luminosity function is limited. The L gamma-LC,5GHz correlation allows us to characterize it starting from the luminosity function of the radio cores, which is modeled with greater accuracy due to the larger number of sources detected at these frequencies. We find that the diffuse emission as extrapolated from the properties of the subthreshold RGs is lower than the one inferred from detected RGs, showing that the contribution of the population of RGs to the IGRB is lower than the previous estimates and is around the similar to 30% level of the IGRB intensity.