WISE properties of AGNs from the AT20G survey

We find counterparts for 88 per cent of 387 compact steep-spectrum (CSS) sources in the AT20G survey, 82+/-5 per cent of which exhibit QSO mid-infrared colours and have moderate redshifts (z_median_=0.82), while those dominated by host galaxy colours in mid-infrared have lower redshifts (z_median_=0.13). The latter classified into late- and early-type galaxies using their mid-infrared colours shows a majority (68+/-4 per cent) have colours characteristic of late-type galaxies.

Thus, we find that a larger fraction of these CSS sources are embedded in hosts with higher gas densities than average early-type galaxies. We compare mid-infrared colours of our AGNs against those reported for AGNs primarily selected using non-radio techniques. This shows that mid-infrared SED of high frequency selected compact radio AGN is comparatively less red, possibly due to contributions from their hosts.