A definitive optical detection of a supercluster at z approximate to 0.91

We present the results from a multiband optical imaging program that has de finitively 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 ori ginally observed in the high-redshift cluster survey of Oke, Postman, & Lub in. They are separated by 4300 km s(-1) in redial velocity and 17' on the p lane of the sky. Their physical and redshift proximity suggested a promisin g supercluster candidate. Deep BRi imaging of the region between the two cl usters 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 spectroscopic ally confirmed early-type galaxies in the two member clusters. The red gala xies are spread throughout the 5 h(-1) Mpc region between Cl 1604 + 4304 an d Cl 1604 + 4321. Their spatial distribution delineates the entire large-sc ale structure with high concentrations at the cluster centers. In addition, we detect a significant overdensity of red galaxies directly between Cl 16 04 + 4304 and Cl 1604 + 4321 which is the signature of a third, rich cluste r associated with this system. The strong sequence of red galaxies and thei r spatial distribution clearly indicate that we have discovered a superclus ter at z approximate to 0.91.