  The classification of the host galaxies of the radio sources in the 25
deg$^2$ ultimate XMM extragalactic survey south field (XXL-S) is presented.
XXL-S was surveyed at 2.1 GHz with the Australia Telescope Compact Array (ATCA)
and is thus far the largest area radio survey conducted down to rms flux
densities of $\sigma \sim 41$ $\mu$Jy beam$^{-1}$. Of the 6287 radio sources in
XXL-S, 4758 (75.7%) were cross-matched to an optical counterpart using the
likelihood ratio technique. There are 1110 spectroscopic redshifts and 3648
photometric redshifts available for the counterparts, of which 99.4% exist out
to $z \sim 4$. A number of multiwavelength diagnostics, including X-ray
luminosities, mid-infrared colours, spectral energy distribution fits, radio
luminosities, and optical emission lines and colours, were used to classify the
sources into three types: low-excitation radio galaxies (LERGs),
high-excitation radio galaxies (HERGs), and star-forming galaxies (SFGs). The
final sample contains 1729 LERGs (36.3%), 1159 radio-loud HERGs (24.4%), 296
radio-quiet HERGs (6.2%), 558 SFGs (11.7%), and 1016 unclassified sources
(21.4%). The LERGs tend to exist in the most massive galaxies with low star
formation rates and redder colours, whereas the HERGs and SFGs exist in
galaxies of lower mass, higher star formation rates, and bluer colours. The
fraction of blue host galaxies is higher for radio-quiet HERGs than for
radio-loud HERGs. LERGs and radio-loud HERGs are found at all radio
luminosities, but radio-loud HERGs tend to be more radio luminous than LERGs at
a given redshift. These results are consistent with the emerging picture in
which LERGs exist in the most massive quiescent galaxies typically found in
clusters with hot X-ray halos and HERGs are associated with ongoing star
formation in their host galaxies via the accretion of cold gas.
