Spectroscopy of faint radio sources: the nature of the sub-mJy radio-source population

Abstract

We have obtained redshift and spectral information for faint radio sources, with S1.4 GHz < 1 mJy, in order to test the hypothesis that the source counts at this flux density are dominated by the distant equivalents of IRAS galaxies. Intermediate-resolution spectra (FWHM = 10–20 Å) were obtained for a magnitude-limited sample (B < 22) of 87 sources from three deep surveys (S1.4 GHz > 0.1 mJy). A large fraction of the observed optical counterparts show emission lines. We discern three populations. (1) Most of the emission-line spectra are consistent with being produced by star formation. The colours, optical luminosities and H α equivalent widths of the star-forming galaxies are similar to those of galaxies identified with faint IRAS sources. (2) A minority of objects have type 1 or type 2 Seyfert emission-line spectra. The broad-line (type 1) objects all have unresolved optical counterparts, and so are QSOs. (3) Some objects show only absorption lines. These are almost all at the bright end of the radio flux density distribution (S1.4 GHz > 1 mJy). Their optical colours and luminosities are similar to those of the elliptical galaxies which dominate radio surveys at higher flux densities. The three populations are well separated in colour-colour-magnitude space, permitting redshift estimates to be made even in the absence of spectral lines. The candidate identifications include a few Galactic stars, but we conclude that most of these are misidentifications. Several clusters are evident in the distributions of redshifts for the galaxies in each field. A few of the clusters include both star-forming and giant elliptical galaxies, confirming that the two populations trace the same distribution of matter on a large scale.