The evolution of faint radio sources

In a previous paper, we presented spectroscopic observations of a complete sample of identifications for faint radio sources S(1.4 GHz) larger than 0.1 mJy and have shown that the majority of the objects are actively star-forming spirals. Here we derive the luminosity functions for elliptical and spiral galaxies in the sample and show that the spiral galaxies are indistinguishable from the starburst galaxy population which dominates the counts at 60 microns. We show by testing a range of different evolutionary assumptions that the faint radio source counts can best be understood as due to the starburst population undergoing strong luminosity evolution at a rate similar to that found for radio galaxies and optical quasars. To reconcile this form of evolution with the faint optical galaxy counts and redshift distribution, it may be necessary to consider the effects of luminosity-dependent dust extinction.