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We have obtained a 12.5 ksec image of the Hercules Cluster, A2151, with the ROSAT PSPC. The sensitivity of the PSPC to low surface brightness features and its large field of view (almost twice the size of the galaxy distribution) enables us to directly compare the large-scale morphology of the gas and galaxy distributions in this spiral-rich irregular cluster.
Comparison of the optical and X-ray data suggest the presence of at least three distinct subclusters in A2151. The brightest X-ray peak coincides with the highest-density peak in the galaxy distribution. As also seen in the Einstein image of this cluster, this peak is bimodal, with no corresponding feature in the galaxy distribution. The northern subclump, distinct in position and velocity, has no associated X-ray gas. The western subclump, apparent in the optical contour map is indistinguishable from the main clump in velocity space and is only a 2$\sigma$ enhancement of the X-ray background, The three group partition of the optical data is a significant improvement over the one group fit, at a $>99.9$\% confidence level.
X-ray spectra derived from the central peak of emission yield a best-fit temperature of 1.6 keV. The emission coincident with the western clump of galaxies is cooler, 0.8 keV, and is outside the 90\% confidence intervals of the central peak temperature.
Within these groups there is little evidence for morphological segregation. This may imply that the subgroups were well-mixed before the merger which formed the rich cluster.
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