3C 40 in Abell 194: can tail radio galaxies exist in quiescent cluster

Abstract

The nearby cluster Abell 194 hosts two luminous, distorted radio galaxies. Both reside within the cluster’s core region, being separated in projection by only 100 kpc. It is often suggested that tailed radio galaxies such as these reside in clusters that are under formation and are accreting new material from their outskirts. In this paper we study the intriguing appearance of Abell 194, and test whether the cluster and radio source dynamics are consistent with the cluster formation/merger model. We analyse data from the XMM-Newton satellite and previously unpublished observations with the Very Large Array (VLA), as well as presenting new data from the Giant Metre-Wave Radio Telescope (GMRT). The shape of the jets, and the lack of significant stripping of the galaxies’ interstellar media, indicate that the radio galaxies are not moving at the large velocities they would have had if they were falling into the cluster from its outskirts; galaxy velocities of 6 300 km s−1 are adequate instead. A plausible scenario that could explain the observations is that the dynamics of the cluster centre are relatively quiescent, with the dominant system of massive galaxies being bound and orbiting the cluster centre of mass. For plausible jet/plume speeds and densities and the galaxy dynamics implied by this picture of the cluster, we show that the observed jet structures can be explained without invoking a major cluster merger event.