Radio Observations of the Hubble Deep Field-South Region. Iv. Optical Properties of the Faint Radio Population

The Australia Telescope Hubble Deep Field-South (ATHDF-S) survey of the Hubble Deep Field-South (HDF-S) reaches sensitivities of ~10 μJy at 1.4, 2.5, 5.2, and 8.7 GHz, making the ATHDF-S one of the deepest surveys ever performed with the Australia Telescope Compact Array (ATCA). Here, we present the optical identifications of the ATHDF-S radio sources using data from the literature. We find that ~66% of the radio sources have optical counterparts to I = 23.5 mag. Deep Hubble Space Telescope (HST) imaging of the area identifies a further 12% of radio sources. We present new spectroscopic observations for 98 of the radio sources and supplement these spectroscopic redshifts with photometric ones calculated from five-band optical imaging. The host galaxy colors and radio-to-optical ratios indicate that low-luminosity (or "radio-quiet") active galactic nuclei make up a significant proportion of the sub-mJy radio population, a result which is in accordance with a number of other deep radio studies. The radio-to-optical ratios of the bright (S _{1.4 GHz} > 1 mJy) sources are consistent with a bimodal distribution.