A definitive optical detection of a supercluster at z ≈ 0.91

We present the results from a multiband optical imaging program that has definitively confirmed the existence of a supercluster at z approximate to 0.91. Two massive clusters of galaxies, Cl 1604 + 4304 at z = 0.924, were originally observed in the high-redshift cluster survey of Oke, Postman, & Lubin. They are separated by 4300 km s(-1) in redial velocity and 17' on the plane of the sky. Their physical and redshift proximity suggested a promising supercluster candidate. Deep BRi imaging of the region between the two clusters indicates a large population of red galaxies. This population forms a tight, red sequence in the color-magnitude diagram at (R - i) approximate to 1.4. The characteristic color is identical to that of the spectroscopically confirmed early-type galaxies in the two member clusters. The red galaxies are spread throughout the 5 h(-1) Mpc region between Cl 1604 + 4304 and Cl 1604 + 4321. Their spatial distribution delineates the entire large-scale structure with high concentrations at the cluster centers. In addition, we detect a significant overdensity of red galaxies directly between Cl 1604 + 4304 and Cl 1604 + 4321 which is the signature of a third, rich cluster associated with this system. The strong sequence of red galaxies and their spatial distribution clearly indicate that we have discovered a supercluster at z approximate to 0.91.