A z=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 similar to 5 x 10(14) M-circle dot. The X-ray masses are consistent with those inferred from optical-X-ray scaling relations established at lower redshift. A strongly lensed z similar to 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 < 3 Mpc in the plane of the sky and likely ! 10 Mpc along the line of sight, and we interpret this structure as the high-redshift antecedent of massive (similar to 10(15) M) clusters such as MS 0451.5 - 0305.