The ATESP Radio Survey IV. Optical Identifications and Spectroscopy in the EIS-A Region

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Abstract
(Abridged) This paper is the fourth of a series reporting the results of the ATESP 1.4 GHz radio survey. The survey covers a region of ~26 sq. degrees at decl. -40 degr. and has produced a catalogue of 2967 radio sources down to ~0.5 mJy (6sigma). In this paper we present the optical identifications over a 3 sq. degr. region of the public ESO Imaging Survey. In this region deep photometry and 95% complete object catalogues in the I band are available down to I~22.5. These data allowed us to identify 57% of the ATESP sources present in the region (219/386). For a magnitude limited sample of 70 identified sources with I1 mJy to 39% at S<1 mJy. Nevertheless, at sub-mJy fluxes, early type galaxies still constitute a significant fraction (25%) of the whole population. We also show that, due to the distribution of radio to optical ratios, sub-mJy samples with fainter spectroscopic follow-ups should be increasingly sensitive to the population of early type galaxies, while a larger fraction of star-forming galaxies would be expected in microJy samples. We compare our results with others obtained from studies of sub-mJy samples and we show how the existing discrepancies can be explained in terms of selection effects.

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