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Title: Chandra and ROSAT observations of Abell 194: detection of an X-ray cavity and mapping the dynamics of the cluster
Authors: Akos Bogdan (1), Ralph P. Kraft (1), William R. Forman (1), Christine Jones (1), Ming Sun (2), Christopher P. O'Dea (3), Eugene Churazov (4), Stefi A. Baum (3) ((1) SAO, (2) University of Virginia, (3) RIT, (4) MPA)

Based on Chandra and ROSAT observations we investigated the nearby poor cluster Abell 194, which hosts two luminous radio galaxies, NGC547 (3C 40) and NGC541 (PKS 0123-016A). We demonstrated the presence of a large X-ray cavity (r ~ 34 kpc) formed by the giant southern radio lobe arising from 3C 40. The estimated age of the cavity is t=7.9 x 10^7 years and the total work of the AGN is 3.3 x 10^59 erg, hence the cavity power is P_cav = 1.3 x 10^44 erg/s. Furthermore, in the Chandra images of NGC545 and NGC541 we detected sharp surface brightness edges, identified as merger cold fronts, and extended tails. Using the pressure ratios between inside and outside the cold fronts we estimated that the velocities of NGC545 and NGC541 correspond to Mach-numbers of M=1.0^{+0.3}_{-0.5} and M=0.9^{+0.2}_{-0.5}, respectively. The low radial velocities of these galaxies relative to the mean radial velocity of Abell 194 imply that their motion is oriented approximately in the plane of the sky. Based on these and earlier observations, we concluded that NGC545 and NGC541 are falling through the cluster, whose center is NGC547, suggesting that Abell 194 is undergoing a significant cluster merger event. Additionally, we detected 20 bright X-ray sources around NGC547 and NGC541, a surprisingly large number, since the predicted number of resolved LMXBs and CXB sources is 2.2 and 4.1, respectively. To explain the nature of additional sources, different possibilities were considered, none of which are satisfactory.

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Title: 3C 40 in Abell 194: can tail radio galaxies exist in a quiescent cluster?
Authors: I. Sakelliou, M. J. Hardcastle, N. N. Jetha

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 <=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.

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