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 ~3 deg² to a 4σ sensitivity of >= 100 μJy at 1.4 GHz, we study the nature of faint radio galaxies. About 50 per cent of the detected radio sources are identified with an 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 radio sources, while spectroscopic observations have been carried out for about 40 per cent of the optically identified sample. These provide redshifts and information on the stellar content. Emission-line ratios imply that most of the emission-line sources are star-forming galaxies, with a small contribution (~ 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-mJy 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 consistent with the V-R and R-K colours of the optically identified sources. For emission-line galaxies, there is a correlation between the radio power and the Hα luminosity, in agreement with the result of Benn et al. This suggests that the radio emission of starburst radio galaxies is a good indicator of star formation activity.