Radio Properties of the Shapley Concentration. II. J1324-3138: a remnant of a radio galaxy in the Abell cluster A3556?

Abstract

In this paper we present a detailed study of the radio galaxy J1324-3138, located at a projected distance of 2 arcmin from the centre of the Abell cluster of galaxies A3556, belonging to the core of the Shapley Concentration, at an average redshift z=0.05. We have observed J1324-3138 over a wide range of frequencies: at 327 MHz (VLA), at 843 MHz (MOST), and at 1376 MHz, 2382 MHz, 4790 MHz and 8640 MHz (ATCA). Our analysis suggests that J1324-3138 is a remnant of a tailed radio galaxy, in which the nuclear engine has switched off and the radio source is now at a late stage of its evolution, confined by the intracluster gas. The radio galaxy is not in pressure equilibrium with the external medium, as it is often found for extended radio sources in clusters of galaxies. We favour the hypothesis that the lack of observed polarised radio emission in the source is due to Faraday rotation by a foreground screen, i.e. the source is seen through a dense cluster gas, characterised by a random magnetic field. An implication of the head-tail nature of the source is that J1324-3138 is moving away from the core of A3556 and that possibly a major merging event between the core of A3556 and the subgroup hosting J1324-3138 has already taken place.