The XXL Survey : XXVI. Optical and near-infrared identifications of the
  ATCA 2.1 GHz radio sources in the XXL-S Field
  In this paper we present the optical, near-infrared (NIR) and X-ray
identifications of the 6287 radio sources detected in the 2.1 GHz deep radio
survey down to a median rms of ~ 41microJy/beam obtained with the Australia
Telescope Compact Array (ATCA) in the XXL-S field. The goal of this paper is to
provide a multi wavelength catalogue of the counterparts of the radio sources
to be used in further studies. For the optical and NIR identification of the
radio sources, we used the likelihood ratio (LR) technique, slightly modified
in order to take into account the presence of a large number of relatively
bright counterparts close to the radio sources. This procedure led to the
identification of optical/NIR counterparts for 4770 different radio sources
(~77% of the whole radio sample), 414 of which also have an X-ray counterpart.
This fraction of identification is in agreement with previous radio-optical
association studies at a similar optical magnitude depth, but is relatively low
in comparison to recent work conducted in other radio fields using deeper
optical and NIR data. The analysis of optical and NIR properties of radio
sources shows that, regardless of the radio flux limit of a radio survey, the
nature of the identified sources is strongly dependent on the depth of the
optical/NIR used in the identification process. Only with deep enough
optical/NIR data will we be able to identify a significant fraction of radio
sources with red (z_{DEC}-K) counterparts whose radio emission is dominated by
nuclear activity rather than starburst activity.
