Accepted to ApJ Preprint typeset using L ATEX style emulateapj v. 11/26/03 SPECTROSCOPIC CONFIRMATION OF THE CL 1604 SUPERCLUSTER AT Z ∼ 0.9

We present spectroscopic confirmation of the Cl 1604 supercluster at z ∼ 0.9. Originally detected as two individual clusters, Cl 1604+4304 at z = 0.90 and Cl 1604+4321 at z = 0.92, which are closely separated in both redshift and sky position, subsequent imaging revealed a complex of red galaxies bridging the two clusters, suggesting that the region contained a large scale structure. We have carried out extensive multi-object spectroscopy, which, combined with previous measurements, provides ∼ 600 redshifts in this area, including 230 confirmed supercluster members. We detect two additional clusters that are part of this structure, Cl 1604+4314 at z = 0.87 and Cl 1604+4316 at z = 0.94. All four have properties typical of local clusters, with line-of-sight velocity dispersions between 489 and 962 km s −1. The structure is significantly extended in redshift space, which, if interpreted as a true elongation in real space, implies a depth of 93 h −1 70 Mpc. We examine the spatial and redshift distribution of the supercluster members. Subject headings: catalogues – surveys – galaxies: clusters: general – large-scale structure of the Universe