A 2572 and HCG 94 - galaxy clusters but not as we know them. An X-ray case study of optical misclassifications

Abstract

We present the results of a spectral-imaging analysis of X-ray data obtained with the Position Sensitive Proportional Counter aboard the ROSAT Observatory in a 32 ks pointed observation of Hickson's Compact Group (HCG) #94. Besides HCG94, A2572, a richness class 0 Abell cluster, is also contained in the central region of the field of view. Both systems are at a redshift of z~0.04 and are falling toward each other at a velocity of about 1000 km/s. Their three-dimensional spatial separation is probably of the order of an Abell radius; however, as yet, no clear signs of dynamical interaction are discernible in the X-ray. We find HCG94's gas temperature and unabsorbed X-ray luminosity to be far too high for a galaxy group thereby confirming the claim of Ebeling, Voges & B"ohringer (1994) that HCG94 should be classified as a galaxy cluster. The opposite is true for the Abell cluster A2572, the optical richness of which has been overestimated due to the inclusion of HCG94. In the X-ray, A2572 appears at first sight like the prototypical binary cluster with two equally massive and X-ray bright subclusters in the process of merging. However, the available X-ray, optical, and radio data strongly suggest that A2572 proper is in fact merely a loose group of galaxies, while the second component is a much richer and more distant cluster seen in superposition. A deprojection analysis shows HCG94 to host a moderate cooling flow; this picture is supported by a radial increase in the column density of absorbing material and a decrease in the gas temperature toward the cluster centre. HCG94's total gravitating mass is much higher than what could be anticipated from its appearance in the optical. Our findings hence underline the need for