Chandra and Rosat observations of A194: Detection of an X-ray cavity and mapping the dynamics of the cluster

Abstract

Based on Chandra and ROSAT observations, we investigated the nearby poor cluster A194, which hosts two luminous radio galaxies, NGC547 (3C 40B) and NGC541 (3C 40A). We demonstrated the presence of a large X-ray cavity (r ∼ 34kpc) formed by the giant southern radio lobe arising from 3C 40B in NGC547. The estimated age of the cavity is t = 7.9 × 107 years and the total work of the active galactic nucleus is 3.3 × 1059 erg, hence the cavity power is P cav = 1.3 × 1044 erg s-1. 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 A194 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 A194 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 low-mass X-ray binaries and cosmic X-ray background sources is 2.2 and 4.1, respectively. To explain the nature of additional sources, different possibilities were considered, none of which are satisfactory. We also studied the origin of X-ray emission in Minkowski's object (MO) and concluded that it is most likely dominated by the population of high-mass X-ray binaries rather than by hot diffuse interstellar medium. Moreover, in view of the galaxy dynamics in A194, we explored the possibility that the starburst in MO was triggered by its past interaction with NGC541, and concluded that it may be a viable path. © 2011. The American Astronomical Society. All rights reserved.