On the morphology of superclusters

A catalog of 48 probable superclusters is generated by means of a percolation technique using a large sample of Abell clusters with measured redshifts z <= 0.1, and various properties of these superclusters are then examined. The shapes and orientations of these superclusters are analyzed using a simple principal axes method, as well as other tests. The superclusters are clearly nonspherical, their shapes being most consistent with a population of predominantly prolate objects having characteristic axial ratios of roughly 3: 1: 1 or 4: 2: 1, although the actual degree of flattening is somewhat uncertain because of the small number of clusters which comprise each supercluster. The orientations of clusters within these superclusters are also examined. Position angle data for clusters have been gathered from many sources in the literature, and supplemented with new measurements presented here. Using only those clusters with reliably determined position angles, it is shown that clusters exhibit a clear tendency to be aligned with one another over scales of at least 30h^-1^ Mpc, and there is strong evidence that they may also be aligned over even larger separations, up to ~ 60h^-1^ Mpc. Taken together, the flattened shapes of the superclusters and the propensity for alignment exhibited by the clusters which reside within them argues strongly in favor of these being genuine physical systems. Possible anisotropies in the orientations of superclusters are also searched for. Neighboring superclusters exhibit no preferred orientations with respect to one another. While there is some hint in the data that superclusters may have a slight tendency to be elongated preferentially along the line of sight, as might result from either large peculiar velocities of clusters within superclusters or projection effects in the Abell catalog, because of the small number of superclusters this cannot be considered statistically significant. Thus the results presented here are most consistent with random orientations of superclusters. Finally, the implications of these results for theories of the formation of large-scale structure in the universe are discussed briefly.