Sources of the Radio Background Considered
  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.
