The VVDS-VLA deep field II. Optical and near infrared identifications of VLA S_1.4GHz > 80 μJy sources in the VIMOS VLT deep survey VVDS-02h field

In this paper we present the optical and near-infrared identifications of the 1054 radio sources detected in the 20 cm deep radio survey down to a 5σ flux limit of ∼80 μJy obtained with the VLA in the VIMOS VLT Deep Survey VVDS-02h deep field. Using U, B, V, R, I and K data, with limiting magnitudes of U_AB ∼ 25.4, B_AB ∼ 26.5, V _AB ∼ 26.2, R_AB ∼ 25.9 I_AB ∼ 25.0, J_AB ∼ 24.2, K_AB ∼ 23.9 (50% completeness) we identified 718 radio sources (∼74% of the whole sample). The photometric redshift analysis shows that, in each magnitude bin, the radio sample has a higher median photometric redshift than the whole optical sample, while the median (V - I)_AB color of the radio sources is redder than the median color of the whole optical sample. These results suggest that radio detection is preferentially selecting galaxies with higher intrinsic optical luminosity. From the analysis of the optical properties of the radio sources as function of the radio flux, we found that while about 35% of the radio sources are optically unidentified in the higher radio flux bin (S > 1.0 mJy), the percentage of unidentified sources decreases to about 25% in the faintest bins (S < 0.5 mJy). The median I_AB magnitude for the total sample of radio sources, i.e. including also the unidentified ones, is brighter in the faintest radio bins than in the bin with higher radio flux. This suggests that most of the faintest radio sources are likely to be associated to relatively lower radio luminosity objects at relatively modest redshift, rather than radio-powerful, AGN type objects at high redshift. Using a classification in early-type and late-type galaxies based on the (B - I)_AB color and the photometric redshift, we found that the majority of the radio sources below ∼0.15 mJy are indeed late-type star forming galaxies. Finally, the radio sources without optical counterpart in our deep imaging have a median radio flux of 0.15 mJy, equal to that of identified sources. Given the very faint optical limits, these unidentified radio sources probably contain a significant fraction of obscured and/or high redshift galaxies.