The radio-mid-infrared correlation and the contribution of 15-μm galaxies to the 1.4-GHz source counts

The radio counterparts to the 15-mum sources in the European Large Area ISO Survey southern fields are identified in 1.4-GHz maps down to similar to80 muJy. The radio-mid-infrared correlation is investigated and derived for the first time at these flux densities for a sample of this size. Our results show that radio and mid-infrared (MIR) luminosities correlate almost as well as radio and far-infrared (FIR), at least up to z similar or equal to 0.6. Using the derived relation and its spread together with the observed 15-mum counts, we have estimated the expected contribution of the 15-mum extragalactic populations to the radio source counts and the role of MIR starburst galaxies in the well-known 1.4-GHz source excess observed at sub-mJy levels. Our analysis demonstrates that IR emitting starburst galaxies do not contribute significantly to the 1.4-GHz counts for strong sources, but start to become a significant fraction of the radio source population at flux densities less than or similar to0.5-0.8 mJy. They are expected to be responsible for more than 60 per cent of the observed radio counts at less than or similar to0.05 mJy. These results are in agreement with the existing results on optical identifications of faint radio sources.