  Recently the ARCADE 2 experiment measured the cosmic radio background (CRB)
and found the brightness temperature of the CRB at 1.4 GHz to be ~480 mK.
Integrating the flux density from the observed 1.4 GHz radio source count
produces a brightness temperature of ~100 mK---less than a quarter of the
observed CRB at 1.4 GHz. Radio quiet AGN are a large fraction of the 1.4 GHz
uJy sources and typically host significant star formation. Thus, it is possible
that AGN and host star formation could be responsible for some fraction of the
excess CRB at 1.4 GHz. Here, an X-ray background population synthesis model is
used in conjunction with empirical radio to X-ray luminosity ratios to
calculate the AGN contribution to the CRB at 1.4 GHz including the emission
from host star formation. It is found that AGN and host star formation
contribute <~9% of the CRB at 1.4 GHz. When all known 1.4 GHz radio source
classes are considered, <~60% of the CRB at 1.4 GHz is accounted for;
therefore, it is necessary that either known radio sources evolve significantly
at flux densities below current survey sensitivity limits or a new population
of low flux density radio sources exist.
