2008ApJ...677L..89G

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 ∼5x10¹⁴ 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 <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 (∼10¹⁵ M_☉) z∼0.5 clusters such as MS 0451.5-0305.