Saraswati: An Extremely Massive ~200 Megaparsec Scale Supercluster

Abstract

Here we report the discovery of an extremely massive and large supercluster (called Saraswati5) found in the Stripe 82 region of SDSS. This supercluster is a major concentration of galaxies and galaxy clusters, forming a wall-like structure spanning at least 200 Mpc across at redshift $z\approx 0.3$. This enormous structure is surrounded by a network of galaxy filaments, clusters, and large, ~40-170 Mpc diameter, voids. The mean density contrast ? (relative to the background matter density of the universe) of Saraswati is gsim1.62 and the main body of the supercluster comprises at least 43 massive galaxy clusters (mean z = 0.28) with a total mass of $\sim 2\times {10}^{16}$ ${M}_{\odot }$. The spherical collapse model suggests that the central region of radius ~20 Mpc and mass at least $4\times {10}^{15}\,{M}_{\odot }$ may be collapsing. This places it among the few largest and most massive superclusters known, comparable to the most massive "Shapley Concentration" ($z\approx 0.046$) in the nearby universe. The Saraswati supercluster and its environs reveal that some extreme large-scale, prominent matter density enhancements formed ~4 Gyr in the past when dark energy had just started to dominate structure formation. This galactic concentration sheds light on the role of dark energy and cosmological initial conditions in supercluster formation, and tests the competing cosmological models.