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

The radio counterparts to the 15-’¦Ìm sources in the European Large Area ISO Survey southern fields are identified in 1.4-GHz maps down to ~80 ’¦ÌJy. 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~= 0.6. Using the derived relation and its spread together with the observed 15-’¦Ìm counts, we have estimated the expected contribution of the 15-’¦Ìm 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 <~0.5-0.8 mJy. They are expected to be responsible for more than 60 per cent of the observed radio counts at <~0.05 mJy. These results are in agreement with the existing results on optical identifications of faint radio sources.