The Complete Local-volume Groups Sample - II. A study of the central radio galaxies in the high-richness sub-sample

We present a study of the radio properties of the dominant early-type galaxies in 26 galaxy groups, the high-richness sub-sample of theComplete Local-volumeGroups Sample (CLoGS). Combining new 610 and 235 MHz observations of 21 groups from the Giant Metrewave Radio Telescope (GMRT) with archival GMRT and Very Large Array (VLA) survey data, we find a high detection rate, with 92 per cent of the dominant galaxies hosting radio sources. The sources have a wide range of luminosities, 1020-1024 WHz-1 in the 235 and 610 MHz bands. Themajority (54 per cent) are point-like, but 23 per cent have radio jets, and another 15 per cent are diffuse radio sources with no clear jet/lobe morphology. Star formation may dominate the radio emission in 2 of the point-like systems and may make a significant contribution to a further 1-3, but is unlikely to be important in the remaining 21 galaxies. The spectral index of the detected radio sources ranges from very flat values of ~0.2 to typical radio synchrotron spectra of ~0.9 with only two presenting steep radio spectra with α235610 > 1. We find that jet sources are more common in X-ray bright groups, with radio non-detections found only in X-ray faint systems. Radio point sources appear in all group environments irrespective of their X-ray properties or spiral fraction.We estimate the mechanical power (Pcav) of the jet sources in the X-ray bright groups to be 1041-1043 erg s-1, with the two large-scale jet systems (NGC 193 and NGC 4261) showing jet powers two orders of magnitude greater than the radiative losses from the cool cores of their groups. This suggests that central AGN are not always in balance with cooling, but may instead produce powerful periodical bursts of feedback heating.