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Paper: The nature of the sub-milliJansky population from a very deep radio and optical survey in the ``Marano Field''
Volume: 176, Observational Cosmology: The Development of Galaxy Systems
Page: 365
Authors: Gruppioni, C.; Mignoli, M.; Zamorani, G.
Abstract: 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 63% of the sources have been optically identified on deep CCD exposures (limit R ≈ 24.0) or ESO 3.6-m plates (limit b_J ≈ 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 fewbroad 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 (with narrow emission-lines in their spectra, consistent with being produced by star-formation activity) at moderate redshift (z < 0.4), with relatively bright magnitudes (B < 22.5) and sub-milliJanski radio fluxes. (3) A group of bright high-redshift (z > 0.8) radio galaxies with moderate-to-strong [OII] emission, B > 22.5 and S_{1.4 GHz} > 1 mJy. 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. Due to the fainter magnitude limit reached in our spectroscopic identifications, even at sub-mJy level the majority of our radio sources are identified with early-type galaxies. Moreover, we conclude that also the great majority of the unidentified radio sources are likely 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.
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