The Phoenix Survey: optical and near-infrared observations of faint radio sources

Using a deep Australia Telescope Compact Array (ATCA) radio survey covering an area of approximate to 3 deg(2) to a 4 sigma sensitivity of greater tha n or equal to 100 mu Jy at 1.4 GHz, we study the nature of faint radio gala xies, About 50 per cent of the detected radio sources are identified with a n optical counterpart revealed by CCD photometry to m(R) == 22.5 mag. Near- infrared (K-band) data are also available for a selected sample of the radi o sources, while spectroscopic observations have been carried out for about 40 per cent of the optically identified sample. These provide redshifts an d information on the stellar content. Emission-line ratios imply that most of the emission-line sources are star-forming galaxies, with a small contri bution (approximate to 10 per cent) from Sy1/Sy2 type objects. We also find a significant number of absorption-line systems, likely to be ellipticals, These dominate at high flux densities (>1 mJy) but are also found at sub-m Jy levels. Using the Balmer decrement we find a visual extinction A(v) = 1. 0 for the star-forming faint radio sources. This moderate reddening is cons istent with the V - R and R - K colours of the optically identified sources , For emission-line galaxies, there is a correlation between the radio powe r and the H alpha luminosity, in agreement with the result of Benn et al. T his suggests that the radio emission of starburst radio galaxies is a good indicator of star formation activity.