Superclustering of faint galaxies in the field of a QSO concentration at z ∼ 1.1

We report on a wide-area (48′ × 9′) imaging survey of faint galaxies in the R and I bands toward the 1338 + 27 field where an unusual concentration of five QSOs at z ∼ 1.1, embedded in a larger scale clustering of 23 QSOs, is known to exist. Using a quite homogeneous galaxy catalog with a detection completeness limit of I ∼ 23.5, we detect a significant clustering signature of faint red galaxies with I > 21 and R - I > 1.2 over a scale extending to ∼20 h^-1₅₀ Mpc. Close examination of the color-magnitude diagram, the luminosity function, and the angular correlation function indeed suggests that those galaxies are located at z ∼ 1.1 and trace the underlying large-scale structure at that epoch, together with the group of five QSOs. Since the whole extent of the cluster of 23 QSOs (∼70 h^-1₅₀ Mpc) is roughly similar to the local " Great Wall," the area may contain a high-redshift counterpart of superclusters in the local universe.