High energy emission component, population, and contribution to the extragalactic gamma-ray background of gamma-ray emitting radio galaxies

TL;DR

This study analyzes the spectra of 61 gamma-ray emitting radio galaxies, constructs their luminosity function, and estimates their contribution to the extragalactic gamma-ray background, revealing many low-redshift sources and a 1-10% contribution.

Contribution

It provides the first gamma-ray luminosity function for radio galaxies and assesses their role in the gamma-ray background, highlighting the prevalence of low-redshift sources.

Findings

Gamma-ray loud radio galaxies are mostly low-redshift.

The gamma-ray luminosity function resembles blazar-like shapes.

Radio galaxies contribute 1-10% to the extragalactic gamma-ray background.

Abstract

In this study, we systematically studied the X-ray to GeV gamma-ray spectra of 61 {\it 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 log$N$-log$S$ 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 $\sim$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.