Radio Galaxies Dominate the High-Energy Diffuse Gamma-Ray Background

It has been suggested that unresolved radio galaxies and radio quasars (sometimes referred to as misaligned active galactic nuclei) could be responsible for a significant fraction of the observed diffuse gamma-ray background. In this study, we use the latest data from the Fermi Gamma-Ray Space Telescope to characterize the gamma-ray emission from a sample of 51 radio galaxies. In addition to those sources that had previously been detected using Fermi data, we report here the first statistically significant detection of gamma-ray emission from the radio galaxies 3C 212, 3C 411, and B3 0309+411B. Combining this information with the radio fluxes, radio luminosity function, and redshift distribution of this source class, we find that radio galaxies dominate the diffuse gamma-ray background, generating $77.2^{+25.4}_{-9.4}\%$ of this emission at energies above $\sim$1 GeV. We discuss the implications of this result and point out that it provides support for scenarios in which IceCube's high-energy astrophysical neutrinos also originate from the same population of radio galaxies.

Comments: 17 pages, 6 figures

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Affiliations: 1Queen's University Belfast, 2Queen's University Belfast, 3Queen's University Belfast, 4Queen's University Belfast, 5Queen's University Belfast, 6Harvard-Smithsonian Center for Astrophysics, 7Ohio University, 8University of California, Santa Cruz, 9Las Cumbres Observatory Global Telescope, 10University of Hawaii at Manoa, 11University of Hawaii at Manoa, 12University of Hawaii at Manoa, 13University of Hawaii at Manoa

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