The contribution of star-forming galaxies to the cosmic radio background

Recent measurements of the temperature of the sky in the radio band, combined with literature data, have convincingly shown the existence of a cosmic radio background with an amplitude of ∼1 K at 1 GHz and a spectral energy distribution that is well described by a power law with index α≃−0.6. The origin of this signal remains elusive, and it has been speculated that it could be dominated by the contribution of star-forming galaxies at high redshift if the far-infrared–radio correlation q(z) evolved in time. We fit observational data from several different experiments by the relation q(z) ≃q0−β log(1 +z) with q0= 2.783 ± 0.024 and β= 0.705 ± 0.081 and estimate the total radio emission of the whole galaxy population at any given redshift from the cosmic star formation rate density at that redshift. It is found that star-forming galaxies can only account for ∼13 per cent of the observed intensity of the cosmic radio background.