Discovery of Extreme Examples of Superclustering in Aquarius

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Abstract
The results of spectroscopic observations of 46 R ≥ 1 clusters of galaxies from the Abell and Abell, Corwin, & Olowin (hereafter ACO) catalogs are presented. The observations were conducted at the ESO 3.6 m telescope with the Meudon-ESO Fibre Optics Spectrograph (MEFOS) multiple-fiber spectrograph. Thirty-nine of the clusters lie in a 10° × 45° strip of sky that contains two supercluster candidates (in Aquarius and Eridanus). These candidates were identified by a percolation analysis of the Abell and ACO catalogs, using estimated redshifts for clusters that had not yet been measured. With our measurements and redshifts from the literature, the target strip is now 85% complete in redshift measurements for R ≥ 1 ACO clusters with m10 ≤ 18.3. Seven other clusters were observed in a supercluster candidate in the Grus-Indus region. Seven hundred thirty-seven galaxy redshifts were obtained in these 46 cluster fields. We find that one of the supercluster candidates is a collection of 14 R ≥ 1 ACO/Abell clusters with a spatial number density that is 20 times the average spatial density for rich ACO clusters. This overdensity has a maximum extent of ~110 h-1 Mpc, making it the longest supercluster composed only of R ≥ 1 clusters to be identified to date. This filament of clusters runs within 6° of the line of sight in the Aquarius region, and, on its high-z end, four R = 0 ACO clusters (three of which are R = 1 in the Abell catalog) appear to bridge gaps to other clusters, extending the structure to ~150 h-1 Mpc. Our analysis also reveals that another supercluster, consisting of eight rich clusters with an extent of ~75 h-1 Mpc, runs roughly perpendicular to Aquarius near its low-redshift end. Both of these superclusters are remarkably filamentary. Fitting ellipsoids to all N ≥ 5 clumps of clusters (at b = 25 h-1 Mpc) in the measured-z Abell/ACO R ≥ 1 clusters sample, we found two other superclusters with axis ratios ≥3 (long-to-midlength axis). The frequency of such filaments (~20%) was nearly identical with that found among "superclusters" in Monte Carlo simulations of random and random-clumped cluster samples, however, so the Abell/ACO clusters have no particular tendency toward filamentation. The Aquarius and Aquarius-Cetus superclusters, in this one region of the sky, have axis ratios of 4.3 and 3.0, respectively. The Aquarius filament also contains a "knot" of six R ≥ 1 clusters at z ~ 0.11, with five of the clusters close enough together to represent an apparent overdensity of 150. There are three other R ≥ 1 cluster density enhancements similar to this knot at lower redshifts: Corona Borealis, the Shapley concentration, and another grouping of seven clusters in Microscopium. All four of these dense superclusters appear near the point of breaking away from the Hubble flow, and some may now be in collapse, but there is little indication of any being virialized. With four such objects, studies of them as a class may now lead to much greater insight into large-scale processes.
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