© CAMBRIDGE UNIVERSITY PRESS 1999

9.2. Superclusters and Pencil-Beam Surveys

Observations of the redshift distribution of galaxies in narrow (~ 40 arcmin) pencil-beam surveys to z 0.3 (Broadhurst et al. 1990; hereafter BEKS) reveal a highly clumped and apparently periodic distribution of galaxies. The distribution features peaks of galaxy counts with an apparently regular separation of 128 Mpc, with few galaxies between the peaks. What is the origin of this clumpy, periodic distribution of galaxies? What does it imply for the nature of the large-scale structure and the properties discussed above? Bahcall (1991) investigated these questions observationally, by comparing the specific galaxy distribution with the distribution of known superclusters.

Bahcall showed that the observed galaxy clumps originate from the tails of large superclusters (Section 9.1). When the narrow-beams intersect these superclusters, which have a mean separation of ~ 100 Mpc, the BEKS galaxy distribution is reproduced.

The redshift distribution of the superclusters is essentially identical to the galaxy redshift distribution, i.e., it reproduces the observed peaks in the BEKS survey, for z 0.1. This indicates that the galaxy clumps observed in the pencil-beam survey originate from these superclusters as the beam crosses the superclusters' surface. The main superclusters that contribute to the clumps were identified. For example, the first northern clump originates from the Coma-Hercules supercluster (= the Great-Wall); the second northern clump is mostly due to the large Corona Borealis supercluster.

The narrow-beam survey of BEKS is directed toward the north and south galactic poles. Some of the Bahcall-Soneira superclusters coincident with the BEKS peaks are located at projected distances of up to ~ 50-100 Mpc from the poles. This suggests that the high-density supercluster regions are embedded in still larger halo surfaces, ~ 100 Mpc in size, and that these large structures surround large underdense regions. The observed number of clumps and their mean separation are consistent with the number density of superclusters and their average extent (Section 9.1).

The narrow widths of the BEKS peaks are consistent with, and imply, flat superclusters. From simulations of superclusters and pencil-beams, Bahcall, Miller, and Udomprasert (1996) find that the observed peak-widths distribution is consistent with that expected of randomly placed superclusters with 15 Mpc width and ~ 150 Mpc extent.

The Bahcall-Soneira superclusters may exhibit weak positive correlations on scales ~ 100-150 Mpc (Bahcall and Burgett 1986). This implies that the superclusters, and thus their related galaxy clumps, are not randomly distributed but are located in some weakly correlated network of superclusters and voids, with typical mean separation of ~ 100 Mpc. This picture is consistent with statistical analyses of the BEKS distribution as well as with the observational data of large-scale structure. The apparent periodicity in the galaxy distribution suggested by BEKS is expected to be greatly reduced when pencil-beams in various directions are combined; the scale reflects the typical mean separation between large superclusters, ~ 100-150h^-1 Mpc, but with large variations at different locations.