Radio Observations of the Hubble Deep Field South Region IV: Optical Properties of the Faint Radio Population

TL;DR

This study presents deep radio observations of the Hubble Deep Field South, identifying optical counterparts and analyzing the properties of faint radio sources, revealing a significant presence of low luminosity AGN and a bimodal distribution in radio-to-optical ratios.

Contribution

It provides the first comprehensive optical identification and spectroscopic analysis of the faint radio population in the Hubble Deep Field South.

Findings

66% of radio sources have optical counterparts to I=23.5 mag

Deep HST imaging identifies an additional 12% of sources

Low luminosity AGN constitute a significant portion of the sub-mJy radio sources

Abstract

The Australia Telescope Hubble Deep Field-South (ATHDFS) survey of the Hubble Deep Field South reaches sensitivities of ~10 miceoJyJy at 1.4, 2.5, 5.2 and 8.7 GHz, making the ATHDFS one of the deepest surveys ever performed with the Australia Telescope Compact Array. Here we present the optical identifications of the ATHDFS radio sources using data from the literature. We find that ~66% of the radio sources have optical counterparts to I = 23.5 mag. Deep 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 5-band optical imaging. The host galaxy colors and radio-to-optical ratios indicate that low luminosity (or "radio quiet") AGN 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_1.4GHz > 1 mJy) sources is consistent with a bimodal distribution.