  Deep imaging and spectroscopy have been carried out for optical counterparts
of a sample of 68 faint radio sources (S > 0.2 mJy) in the ``Marano Field''.
About 60% of the sources have been optically identified on deep CCD exposures
(limit R ~ 24.0) or ESO 3.6-m plates (limit bJ ~ 22.5). Thirty-four spectra
(50% of the total radio sample) were obtained with the ESO 3.6-m telescope and
30 redshifts were determined. In addition to a few broad line active galactic
nuclei, three main spectroscopic classes have been found to dominate the faint
radio galaxy population: (1) Early-type galaxies with 0.1 < z < 0.8. (2)
Late-type galaxies at moderate redshift (z < 0.4), with relatively bright
magnitudes (B < 22.5) and sub-mJy radio fluxes. (3) A group of bright
high-redshift (z > 0.8) radio galaxies with moderate-to-strong [OII] emission.
They have spectra, colours and absolute magnitudes similar to those of the
classical bright elliptical radio galaxies found in surveys carried out at
higher radio fluxes. Star-forming galaxies do not constitute the main
population of our radio sources identified with galaxies. In fact, even at
sub-mJy level the majority of our radio sources are identified with early-type
galaxies. This apparent discrepancy with previous results is due to the fainter
magnitude limit reached in our spectroscopic identifications. Moreover, using
mainly the large radio-to-optical ratio and the information from the available
limits on the optical magnitudes of the unidentified radio sources, we conclude
that the great majority of them are likely to be early-type galaxies, at z > 1.
If correct, it would suggest that the evolution of the radio luminosity
function of spiral galaxies, including starbursts, might not be as strong as
suggested in previous evolutionary models.
