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 nucleimake up a significantproportion of the sub-mJy radio population, a resultwhich is in accordance with a number of other deep radio studies. The radio-to-optical ratios of the bright (S1.4 GHz > 1 mJy) sources are consistent with a bimodal distribution