SIMBAD references

By combining the REFLEX and NORAS cluster data sets with the NVSS radio catalogue, we obtain a sample of 145, z < 0.3, X-ray-selected clusters brighter than 3x10^–12 erg/s/cm2 that show a central radio emission above 3 mJy. For virial masses M_vir≲ 10^14.5M_☉, 11 clusters out of 12 (corresponding to 92 per cent of the systems) are inhabited by a central radio source. This fraction decreases with higher masses as ∝ M^–0.4_vir. If this decrease is a selection effect, it suggests that the majority of X-ray-selected clusters host in their centre a radio source brighter than ∼10²⁰ W/Hz/sr. A division of the sample into clusters harbouring either point-like or an extended radio-loud active galactic nucleus (AGN) reveals that the steepening of the L_X-T relation for low-temperature clusters is strongly associated with the presence of central radio objects with extended jets and/or lobe structures. In the latter case, L_X∝ T⁴ while for point-like sources one recovers an approximately self-similar relation L_X∝ T^2.3. Monte Carlo simulations show that the steepening of the L_X-T relation is not caused by clusters being underluminous in the X-ray band, but rather by overheating, most likely caused by the interplay between the extended radio structures and the intracluster medium. In the case of low-mass systems, we also find a tight correlation between radio luminosity and cluster temperature. The effects of the central radio source on the thermal state of a cluster become less important with increasing cluster mass.

The presence of radio sources with extended structures (61, corresponding to ∼42 per cent of the sample) is enhanced in X-ray luminous clusters with respect to `field' radio-loud AGN. Furthermore, we find that the luminosity distribution of the cluster radio population differs from that of all radio sources, as there is a deficit of low-luminosity (L_R≲ 10²² W/Hz/sr) objects, while the number of high-luminosity ones is boosted. The net effect on the radio luminosity function of radio galaxies associated with cluster centres is of a flattening at all luminosities L_R≲ 10²⁴ W/Hz/sr.