Radio properties of the Shapley concentration - III. Merging clusters in the A3558 complex

Abstract

We present the results of a 22-cm radio survey carried out with the Australia Telescope Compact Array (ATCA) covering the A3558 complex, a chain formed by the merging ACO clusters A3556-A3558-A3562 and the two groups SC 1327-312 and SC 1323-313, located in the central region of the Shapley Concentration. The purpose of our survey is to study the effects of cluster mergers on the statistical properties of radio galaxies and to investigate the connection between mergers and the presence of radio haloes and relic sources. We found that the radio source counts in the A3558 complex are consistent with the background source counts. The much higher optical density compared with the background is not reflected as a higher density of radio sources. Furthermore, we found that no correlation exists between the local density and the radio source power, and that steep-spectrum radio galaxies are not segregated in denser optical regions. The radio luminosity function for elliptical and S0 galaxies is significantly lower than for cluster early-type galaxies and for those not selected to be in clusters at radio powers log P1.4 ≳ 22.5, implying that the probability of a galaxy becoming a radio source above this power limit is lower in the Shapley Concentration compared with any other environment. Possible explanations will be presented. The detection of a head-tail source in the centre of A3562, coupled with careful inspection of the 20-cm NRAO VLA Sky Survey (NVSS) and of 36-cm MOST observations, allowed us to spot two extended sources in the region between A3562 and SC 1329-313, i.e. a candidate radio halo at the centre of A3562 and low brightness extended emission around a 14.96-mag Shapley galaxy. The relation between these two extended galaxies and the ongoing group merger in this region of the Shapley Concentration are discussed.