Proc. Natl. Acad. Sci. USA Vol. 90, pp. 4848-4852, June 1993 Colloquium Paper This paper was presented at a colloquium entitled "Physical Cosmology, " organized by a committee chaired by David N. Schramm, held March 27 and 28, 1992, at the National Academy of Sciences, Irvine, CA. Clusters, superclusters, and large-scale structure: A consistent picture NETA A. BAHCALL Astrophysical Sciences, Princeton University, Princeton, NJ 08544-1001 ABSTRACT Observations of the large-scale structure in tary in shape. Gregory and Thompson (5) obtained a redshift the universe using different tracers and techniques, including survey of galaxies in the direction ofthe Coma cluster. They the spatial distribution of galaxies, clusters of galaxies, narrow found the large, flattened Coma supercluster, which is part of pencil-beam surveys, and quasars, appear to be yielding a the recently named "Great Wall," extending to at least %40 consistent picture of the universal structure. A network of Mpc. The supercluster surrounds a large underdense region large-scale superclusters with scales up to -l50h-1 Mpc is of comparable size. Additional surveys (6, 7) yielded similar suggested (where h - 0.5-1 is the Hubble constant in units of results in the Hercules and Perseus superclusters. More 100 km-s"Mpc'1; 1 pc = 3.09 x 1016 m; h = 1 is used recent galaxy redshift surveys (8-10) revealed similar large- throughout this paper). The supercluster network surrounds scale low-density regions, suggesting a "cellular" structure of the superclusters surrounding low-density regions. universe. The universal dimensionless cluster correlation func- Large-scale superclusters have been traced very success- tion, supported by new data from automated cluster surveys, fully by rich clusters of galaxies (4, 11, 12). A complete is consistent with this picture. The "standard" Qi = 1 cold dark catalog of superclusters-defined as clusters of clusters of matter (CDM) model for the universe appears to be inconsistent galaxies-was constructed by Bahcall and Soneira (11) from with the details of the observed large-scale structure distribu- a complete redshift sample of rich Abell (4) clusters to z < tion; a low-density, Q1 0.2-0.3, CDM model provides a 0.08. The catalog identifies all superclusters that have a considerably better fit to the observations. spatial density enhancementf2 20 times larger than the mean cluster density. The mean density of the Bahcall-Soneira Statistical analysis of the spatial distribution of galaxies (1) (BS) superclusters is ::=10-6 Mpc-3, with an average mean suggests that, on average, the galaxy distribution is uniform supercluster separation of %100 Mpc. The superclusters on scales larger than -20h-1 Mpc. At the same time the contain a large fraction of all clusters: -54% atf - 20. The existence of some specific large-scale structure in the uni- linear size of the largest superclusters is =150 Mpc (e.g., verse has been known for over half a century. Shapley (2) Corona Borealis) and they are elongated in shape. The noticed a large remote "cloud of galaxies" in Centaurus, fractional volume of space occupied by the superclusters is known today as the Shapley Supercluster: an %50-Mpc very small: -3% at f - 20. Similar results were recently structure that is rich and dense in clusters of galaxies (3). obtained by Postman et al. (12). Zwicky (49) noticed the very large galaxy concentration in A redshift-cone diagram of the superclusters in the decli- Pisces that also encompasses several clusters. Abell (4) nation slice 8 = 0o-40° has been presented by Bahcall (13). recognized that rich clusters of galaxies were themselves The mean separation of the superclusters, =100 Mpc, is clustered into second-order clustering-i.e., superclusters. apparent. We shall also see below that the superclusters The scales of the above superclusters reached tens of mega- surround large, low-density regions such as the Bootes void parsecs. (between the Hercules and Corona Borealis superclusters) as Over the last decade, a great deal has been learned about well as underdense regions seen in pencil-beam surveys. the large-scale structure in our nearby universe. While the How do these superclusters compare with the structures specific nature of the structure, its topology, and its extent found by galaxy redshift surveys? Superimposing the super- are not fully understood, a consistent picture is emerging cluster contours on the cumulative galaxy redshift map from from different types of observations: the spatial distribution the Center for Astrophysics (CfA) survey (14) shows that the ofgalaxies, clusters ofgalaxies, narrow pencil-beam surveys, superclusters identified by the clustering ofclusters highlight as well as the distribution ofquasars and active galactic nuclei well the main large-scale systems seen in the galaxy survey. (AGNs). The consistent picture suggests a network of large- In particular, the union ofthe Coma and Hercules superclus- scale superclusters, up to 100-150 Mpc in scale, that sur- ters constitutes the Great Wall seen in the CfA survey; the rounds lower-density regions. A "cellular" structure of the Great Wall is thus a merging of two BS superclusters, with a universe, similar to the "pancake" model discussed by total extent of =150 Mpc and thickness of 510 Mpc. This Zeldovich and collaborators (50), is suggested by the data. extent and flattened shape are comparable to those of the These data provide critical constraints to possible cosmolog- other large superclusters in the BS catalog; for example, the ical models of the universe. Corona Borealis supercluster is another such Great Wall, considerably greater and richer than Coma-Hercules. It is Superclusters located behind the large void in Bootes. This comparison of the galaxy and cluster distribution indicates that the large- Early redshift surveys of galaxies have already revealed that scale structures traced by both galaxies and rich clusters are superclusters are large systems that are flattened or filamen- Abbreviations: AGN, active galactic nuclei; APM, automated plate The publication costs of this article were defrayed in part by page charge measuring machine; BS, Bahcall-Soneira; BEKS, Broadhurst et al.; payment. This article must therefore be hereby marked "advertisement" CDM, cold dark matter; CfA, Center for Astrophysics; EDCC, in accordance with 18 U.S.C. §1734 solely to indicate this fact. Edinburgh-Durham Cluster Catalog. 4848 Downloaded by guest on September 29, 2021 Colloquium Paper: Bahcall Proc. Natl. Acad. Sci. USA 90 (1993) 4849 consistent with each other; both find the same superclusters. ond northern clump is mostly due to the large Corona While the rich clusters are most efficient in finding the Borealis supercluster (BS 12) (13). largest-scale structures, the galaxies trace the small-scale The narrow-beam survey of BEKS is directed toward the connectedness to the larger scales. north and south galactic poles. Some of the BS superclusters A different method of finding superclusters is that used by coincident with the BEKS peaks are located at projected Lynden-Bell et al. (15), who utilized peculiar velocities to distances of up to 50-100 Mpc from the poles. This suggests, infer the existence of massive superclusters such as the Great similar to the Bootes void analysis, that the high-density Attractor. The estimated mass of the Great Attractor, -5 x supercluster regions are embedded in larger halo surfaces, 1016 times the mass of the sun (Mo) (15), is comparable to that =100 Mpc in size, and that these large structures surround of the large BS superclusters. The Great Attractor does not large underdense regions. The observed number of clumps appear, however, to contain rich clusters. and their mean separation are consistent with the number We thus see that clusters, galaxies, and velocity fields, as density of superclusters and their average extent. well as pencil-beam surveys (see below), appear to trace The narrow widths of the BEKS peaks are consistent with, similar superclusters. These superclusters are the largest and imply, flat superclusters. Simulations of superclusters systems yet observed. Their sizes extend to -1502 x 20 and pencil beams (21) showed that the observed peak-widths Mpc3, and their mass is estimated to be 2-10 x 1016M® (16). distribution is consistent with that expected of randomly This mass is comparable to the mass of 20-50 rich clusters. placed superclusters with s20-Mpc width (and =150-Mpc There are some indications that the supercluster distribution extent). The apparent periodicity in the galaxy distribution is is not random (17), suggesting positive correlations among expected to be greatly reduced when pencil beams in various superclusters on the 100- to 150-Mpc scale. directions are combined. Superclusters Around Voids Cellular Model of Large-Scale Structure The observational data described above suggest a "cellular" The area around the large, -60-Mpc-diameter void of galax- structure in the universe (e.g., a Zeldovich "pancake" mod- ies in Bootes (18) was studied (19) using the BS supercluster el), in which large-scale flattened superclusters surround catalog. The largest, densest superclusters are located near low-density regions. Such a model was simulated (22) with and around the area devoid ofgalaxies (-'14.5h + 500). In the galaxies placed on surfaces of randomly placed shells, and redshift-cone diagram (13), the void is located between clusters placed at shell intersections. This model produced Hercules (part of the Great Wall) and