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We present the results of ROSAT pointed observations of the poor clusters MKW2, N79--299A, S49--128, S49--132 and S49--140. These poor clusters all contain extended radio sources, generally with a bent, head-tail (HT) morphology. It has been thought that HTs should only be found in rich clusters, which have suffciently high intracluster medium (ICM) densities and velocity dispersions for effective ram pressure bending of the radio jets. Yet, at least a dozen HTs are known to exist in poor clusters. This situation has motivated us to perform ROSAT pointed observations of the ICM in 5 poor clusters with HTs. We have found that the X-ray emission associated with these clusters is generally quite clumpy and asymmetric. Often, the clumps are associated with subgroups of galaxies and individual galaxies, as well as with radio sources.
We hypothesize that many poor clusters have recently collapsed out of large, loose clouds of galaxies. We believe that several of our results are understandable in light of this hypothesis. First, the observed ICM densities in these clusters, as well as their unusually broad velocity distributions, provide enough ram pressure to bend the radio jets. Second, some of these galaxies (such as NGC 4061, within N79--299A) show signs of interactions with neighboring galaxies. Third, 4 of these 5 clusters are embedded within larger Zwicky clusters. This may indicate that Zwicky's nearby, open clusters are in fact real systems and act as ``incubators'' of poor clusters. Fourth, the flat, broad velocity distributions may reflect the velocities associated with the larger scale systems from which we hypothesize that these poor clusters have collapsed. We believe that this hypothesis has sufficient merit to warrant further investigation.
