We investigate different scenarios for the origin of the extragalactic radio background. The surface brightness of the background, as reported by the Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission 2 (ARCADE 2) collaboration, is several times higher than that which would result from currently observed radio sources. We consider contributions to the background from diffuse synchrotron emission from clusters and the intergalactic medium, previously unrecognized flux from low-surface-brightness regions of radio sources and faint point sources below the flux limit of existing surveys. By examining radio source counts available in the literature, we conclude that most of the radio background is produced by radio point sources that dominate at sub-μJy fluxes. We show that a truly diffuse background produced by electrons far from galaxies is ruled out because such energetic electrons would overproduce the observed X-ray/γ -ray background through inverse Compton scattering of the other photon fields. Unrecognized flux from low-surface-brightness regions of extended radio sources, or moderate flux sources missed entirely by radio source count surveys, cannot explain the bulk of the observed background but may contribute as much as 10 per cent. We consider both radio supernovae and radio-quiet quasars as candidate sources for the background and show that both fail to produce it at the observed level because of an insufficient number of objects and total flux, although radio-quiet quasars contribute at the level of at least a few per cent. We conclude that if the radio background is at the level reported, a majority of the total surface brightness would have to be produced by ordinary star-forming galaxies above redshift 1 characterized by an evolving radio–far-infrared correlation, which changes towards the radio loud with redshift.

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Copyright © 2010 SLAC National Accelerator Laboratory. This article first appeared in Monthly Notices of the Royal Astronomical Society 409, no. 3 (December 11, 2010): 1172-182. doi:10.1111/j.1365-2966.2010.17382.x.

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