H. Ebeling et al., A-2572 AND HCG-94 - GALAXY CLUSTERS BUT NOT AS WE KNOW THEM - AN X-RAY CASE-STUDY OF OPTICAL MISCLASSIFICATIONS, Monthly Notices of the Royal Astronomical Society, 277(3), 1995, pp. 1006-1032
We present the results of a spectral-imaging analysis of X-ray data ob
tained with the Position Sensitive Proportional Counter aboard the ROS
AT Observatory in a 32-ks pointed observation of Hickson's Compact Gro
up (HCG) # 94. Besides HCG 94, A 2572, a richness class 0 Abell cluste
r, 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-dimension
al spatial separation is probably of the order of an Abell radius; how
ever, as yet, no dear signs of dynamical interaction are discernible i
n the X-ray data. We find HCG 94's gas temperature and unabsorbed X-ra
y luminosity to be far too high for a galaxy group, thereby confirming
the claim of Ebeling, Voges & Bohringer that HCG 94 should be classif
ied 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 sub
clusters in the process of merging. However, the available X-ray, opti
cal, and radio data strongly suggest that A2572 proper is, in fact, me
rely a loose group of galaxies, while the second component is a much r
icher and more distant cluster seen in superposition. A deprojection a
nalysis shows HCG 94 to host a moderate cooling flow; this picture is
supported by a radial increase in the column density of absorbing mate
rial and a decrease in the gas temperature toward the cluster centre.
HCG 94's total gravitating mass is much higher than what could be anti
cipated from its appearance in the optical. Our findings hence underli
ne 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 th
e overall baryon fraction of the Universe, then this result requires t
he latter to be open with Q(0) < 0.35 if a conflict with the baryon de
nsity derived from nucleosynthesis calculations is to be avoided.