A study of extended radio galaxies in the Shapley concentration core

Extended cluster radio galaxies show different morphologies com- pared to those found isolated in the field. Indeed, symmetric double radio galaxies are only a small percentage of the total content of ra- dio loud cluster galaxies, which show mainly tailed morphologies (e.g. O’Dea & Owen, 1985). Moreover, cluster mergers can deeply affect the statistical properties of their radio activity. In order to better understand the morphological and radio activity differences of the radio galaxies in major mergeing and non/tidal-merging clusters, we performed a multifrequency study of extended radio galax- ies inside two cluster complexes, A3528 and A3558. They belong to the innermost region of the Shapley Concentration, the most massive con- centration of galaxy clusters (termed supercluster) in the local Universe, at average redshift z ≈ 0.043. We analysed low frequency radio data performed at 235 and 610 MHz with Giant Metrewave Radio Telescope (GMRT) and we combined them with proprietary and literature observations, in order to have a wide frequency range (150 MHz to 8.4 GHz) to perform the spectral analysis. The low frequency images allowed us to carry out a detailed study of the radio tails and diffuse emission found in some cases. The results in the radio band were also qualitatively compared with the X-ray information coming from XMM-Newton observations, in order to test the interaction between radio galaxies and cluster weather. We found that the brightest central galaxies (BCGs) in the A3528 cluster complex are powerful and present substantial emission from old relativistic plasma characterized by a steep spectrum (α > 2). In the light of observational pieces of evidence, we suggest they are possible re-started radio galaxies. On the other hand, the tailed radio galaxies trace the host galaxy motion with respect to the ICM, and our find- ings is consistent with the dynamical interpretation of a tidal interaction (Gastaldello et al. 2003). On the contrary, the BCGs in the A3558 clus- ter complex are either quiet or very faint radio galaxies, supporting the hypothesis that clusters mergers quench the radio emission from AGN.

Radio halos in a mass-selected sample of galaxy clusters

A fraction of galaxy clusters host Mpc-scale Radio Halos (RH), generated by ultrarelativistic electrons in the magnetized intra cluster medium (ICM). In the current view they trace turbulent regions in merging clusters, where relativistic particles are trapped and accelerated. This model has clear expectations about the statistical properties of RHs. To test these expectations large mass-selected samples of galaxy clusters with adequate radio and X-ray data are necessary. We used the Planck SZ cluster catalogue as suitable starting point of our investigation, selecting clusters with M500>6x10^14 Msun at 0.08and with the Extended GMRT RH Survey for 0.2and show that this increase is in line with statistical calculations based on the re-acceleration scenario. We confirm earlier findings that RHs are exclusively found in merging systems.