X-ray and optical observations of the Shapley supercluster in Hydra–Centaurus

X-ray and optical observations of the ‘Shapley Supercluster’, at redshifts between 0.037 and 0.053 in the region of Hydra and Centaurus, are presented. The X-ray data show that many of the clusters are multiple and confirm the exceptionally high density of rich clusters. Six of the 46 X-ray brightest clusters at high galactic latitude belong to this supercluster. The Local Supercluster is less luminous in X-rays than the leastluminous cluster found here, and only the Centaurus cluster in the Hydra-Centaurus Supercluster (the Great Attractor) is (just) more luminous than the weakest cluster detected in the Shapley Supercluster. The optical and X-ray luminosity functions suggest that there are large deviations from the Hubble flow in the region and that clusters are merging at a high rate. The minimum mass of the supercluster, obtained by summing the masses of the separate clusters estimated from their X-ray luminosity, exceeds $$1.4\times{10}^{16}h_{50}^{-1}\enspace {M}_{\odot}$$. This is consistent with the mass estimated from opticaldipole measurements. The ratio of mass density in observed clusters to the mean density of the Universe exceeds $$0.8\enspace \Omega_{0}^{-1}$$ over the core region ofradius $$37\enspace h_{50}^{-1}$$.