Sources of the Radio Background Considered

TL;DR

This paper explores various potential sources of the extragalactic radio background, concluding that most of it likely originates from faint radio point sources associated with star-forming galaxies at high redshift.

Contribution

It systematically evaluates and rules out diffuse emission, low surface brightness regions, and known faint sources as primary origins, proposing high-redshift star-forming galaxies as the main contributors.

Findings

Diffuse synchrotron emission is insufficient to explain the background.

Unrecognized flux from low surface brightness regions accounts for up to 10%.

Star-forming galaxies at redshift >1 likely dominate the radio background.

Abstract

We investigate different scenarios for the origin of the extragalactic radio background. The surface brightness of the background, as reported by the 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 microJy fluxes. We show that a truly diffuse background produced by electrons far from galaxes is ruled out because such energetic electrons would overproduce the obserevd X-ray/gamma-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 insufficient number of objects and total flux, although radio quiet quasars contribute at the level of at least a few percent. 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 starforming galaxies above redshift 1 characterized by an evolving radio far-infrared correlation, which changes toward the radio loud with redshift.