2003A&A...398..901C

We have obtained a deep radio image with the Very Large Array at 6 cm in the Lockman Hole. The noise level in the central part of the field is ∼11µJy. From these data we have extracted a catalogue of 63 radio sources with a maximum distance of 10arcmin from the field center and with peak flux density greater than 4.5 times the local rms noise. The differential source counts are in good agreement with those obtained by other surveys. The analysis of the radio spectral index suggests a flattening of the average radio spectra and an increase of the population of flat spectrum radio sources in the faintest flux bin. Cross correlation with the ROSAT/XMM X-ray sources list yields 13 reliable radio/X-ray associations, corresponding to ∼21% of the radio sample. Most of these associations (8 out of 13) are classified as type II AGN. Using optical CCD (V and I) and K' band data with approximate limits of V∼25.5mag, I∼24.5mag and K'∼20.2mag, we found an optical identification for 58 of the 63 radio sources. This corresponds to an identification rate of ∼92%, one of the highest percentages so far available. From the analysis of the colour-colour diagram and of the radio flux - optical magnitude diagram we have been able to select a subsample of radio sources whose optical counterparts are likely to be high redshift (z>0.5) early-type galaxies, hosting an Active Galactic Nucleus responsible of the radio activity. This class of objects, rather than a population of star-forming galaxies, appears to be the dominant population (>50%) in a 5GHz selected sample with a flux limit as low as 50µJy. We also find evidence that at these faint radio limits a large fraction (∼60%) of the faintest optical counterparts (i.e. sources in the magnitude range 22.5<I<24.5mag) of the radio sources are Extremely Red Objects (EROs) with I-K'>4 and combining our radio data with existing ISO data we conclude that these EROs sources are probably associated with high redshift, passively evolving elliptical galaxies. The six radio selected EROs represent only ∼2% of the optically selected EROs present in the field. If their luminosity is indeed a sign of AGN activity, the small number of radio detections suggests that a small fraction of the EROS population contains an active nucleus.