Abstract

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 × 10 14 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. © 2008. The American Astronomical Society. All right reserved.