A-2572 AND HCG-94 - GALAXY CLUSTERS BUT NOT AS WE KNOW THEM - AN X-RAY CASE-STUDY OF OPTICAL MISCLASSIFICATIONS

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 HCG 94, A 2572, 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 similar to 0.04 and are falling toward each other at a velocity of about 1000 km s(-1). Their three-dimensional spatial separation is probably of the order of an Abell radius; however, as yet, no dear signs of dynamical interaction are discernible in the X-ray data. We find HCG 94's gas temperature and unabsorbed X-ray luminosity to be far too high for a galaxy group, thereby confirming the claim of Ebeling, Voges & Bohringer that HCG 94 should be classified as a galaxy cluster. The opposite is true for the Abell cluster A 2572, the optical richness of which has been overestimated due to the inclusion of HCG 94. In the X-ray, A 2572 appears at first sight like a typical 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 HCG 94 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. HCG 94'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 X-ray-selected cluster samples. For all three clusters studied in this paper we find the baryon fraction to rise with radius and reach values of 15 to 30 per cent at the outer edge of our study regions. If any of these values is to be taken as representative of the overall baryon fraction of the Universe, then this result requires the latter to be open with Q(0) < 0.35 if a conflict with the baryon density derived from nucleosynthesis calculations is to be avoided.