A <i>z</i> = 0.9 Supercluster of X-Ray Luminous, Optically Selected, Massive Galaxy Clusters

We report the discovery of a compact supercluster structure at z = 0.9. The structure comprises three optically selected clusters, all of which are detected in X-rays and spectroscopically confirmed to lie at the same redshift. The Chandra X-ray temperatures imply individual masses of ~5 × 1014 M☉. The X-ray masses are consistent with those inferred from optical-X-ray scaling relations established at lower redshift. A strongly lensed z ~ 4 Lyman break galaxy behind one of the clusters allows a strong-lensing mass to be estimated for this cluster, which is in good agreement with the X-ray measurement. Optical spectroscopy of this cluster gives a dynamical mass in good agreement with the other independent mass estimates. The three components of the RCS 2319+00 supercluster are separated from their nearest neighbor by a mere &lt;3 Mpc in the plane of the sky and likely &lt;10 Mpc along the line of sight, and we interpret this structure as the high-redshift antecedent of massive (~1015 M☉) z ~ 0.5 clusters such as MS 0451.5–0305.

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