The ATESP 5 GHz radio survey - II. Physical properties of the faint radio population

Context. One of the most debated issues about sub-mJy radio sources, which are responsible for the steepening of the 1.4 GHz source counts, is the origin of their radio emission. Particularly interesting, from this point of view, is the possibility of combining radio spectral index information with other observational properties to assess whether the sources are triggered by star formation or nuclear activity. Aims. The aim of this work is to study the optical and near infrared properties of a complete sample of 131 radio sources with S > 0.4 mJy, observed at both 1.4 and 5 GHz as part of the ATESP radio survey. The availability of multi-wavelength radio and optical information is exploited to infer the physical properties of the faint radio population. Methods. We use deep multi-colour (UBVRIJK) images, mostly taken in the framework of the ESO Deep Public Survey, to optically identify and derive photometric redshifts for the ATESP radio sources. Deep optical coverage and extensive colour information are available for 3/4 of the region covered by the radio sample. Typical depths of the images are U similar to 25, B similar to 26, V similar to 25.4, R similar to 25.5, I similar to 24.3, 19.5 <= K-s <= 20.2, J <= 22.2. We also add shallower optical imaging and spectroscopy obtained previously in order to perform a global analysis of the radio sample. Results. Optical/near infrared counterparts are found for similar to 78% (66/85) of the radio sources in the region covered by the deep multicolour imaging, and for 56 of these reliable estimates of the redshift and type are derived. We find that many of the sources with flat radio spectra are characterised by high radio-to-optical ratios (R > 1000), typical of classical powerful radio galaxies and quasars. Flat-spectrum sources with low R values are preferentially identified with early type galaxies, where the radio emission is most probably triggered by low-luminosity active galactic nuclei. Considering both early type galaxies and quasars as sources with an active nucleus, such sources largely dominate our sample (78%). Flat-spectrum sources associated with early type galaxies are quite compact (d < 10 - 30 kpc), suggesting core-dominated radio emission.