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 (LAT) 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 logN -logS relation prefers models with more low-z radio galaxies. This indicates many low-z gamma-ray-loud radio galaxies. By 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.

The Fourth Catalog of Active Galactic Nuclei Detected by the Fermi Large Area Telescope -- Data Release 3

“…FSRQs preferentially exhibit E IC pk below 1 GeV, while most BL Lacs have E IC pk above this value, reflecting the dichotomy seen in the photon-index distributions (Figure 1). Most values of E IC pk obtained for the 9 radio galaxies meeting the above condition are notably larger than those reported from a combined fit to the X-ray and gamma-ray data (see e.g., Fukazawa et al 2022). An explanation for this discrepancy can be the presence of a prominent emission from the corona/accretion disk in the X-ray to soft gamma-ray bands, which drives down the fitted E IC pk value in the latter case.…”