High-energy Emission Component, Population, and Contribution to the Extragalactic Gamma-Ray Background of Gamma-Ray-emitting Radio Galaxies

Abstract

In this study, we systematically studied the X-ray to GeV gamma-ray spectra of 61 Fermi Large Area Telescope detected radio galaxies. We found an anticorrelation between peak frequency and peak luminosity in the high-energy spectral component of radio galaxies, similar to blazars. With this sample, we also constructed a gamma-ray luminosity function (GLF) of gamma-ray-loud radio galaxies. We found that blazar-like GLF shapes can reproduce their redshift and luminosity distribution, but the log N-log S relation prefers models with more low-z radio galaxies. Utilizing our latest GLF, the contribution of radio galaxies to the extragalactic gamma-ray background is found to be 1%-10%. We further investigated the nature of gamma-ray-loud radio galaxies. Compared to radio or X-ray flux-limited radio galaxy samples, the gamma-ray-selected sample tends to lack high radio power galaxies like FR II radio galaxies. We also found that only ~10% of radio galaxies are GeV gamma-ray loud. Radio galaxies may contribute to the cosmic MeV gamma-ray background comparable to blazars if gamma-ray-quiet radio galaxies have X-ray to gamma-ray spectra like Cen A, with a small gamma-ray-to-X-ray flux ratio.