Annu. Rev. Astron. Astrophys. 1988. 26: 245-294 Copyright © 1988 by Annual Reviews. All rights reserved

2.1.3. STRUCTURE OF CONTIGUOUS SHELLS

The structure of superclusters, the material entities that make up the contiguous shells, is reviewed by Oort (128). Particularly interesting are correlations of structural properties of galaxies such as Hubble morphological type with local number density of galaxies, first discovered by Dressler (cf. 63a) for rich clusters and later found to extend monotonically into the domain of the dispersed component of superclusters (41, 84b).

The determination of the geometry of contiguous shells is basically a problem in map making [cf. Robinson & Sale (150) and MacKay (114)]. Ellis et al. (67) review the theoretical foundations of established astronomical methods for determining the structure of the Universe from telescopic observations. They also discuss selection effects introduced by this process. Barrow & Bhavsar (22a) examine factors that determine pattern recognition by the human eye, with special application to their role in the evaluation of whether large-scale structure is, for example, filamentary or cellular.

The determination of geometry of contiguous shells derived by identifying observed Doppler velocities of galaxies with their Hubble velocities is systematically distorted through the neglect of corrections for motions relative to the Hubble flow. The latter include (a) the motion of a galaxy in a system relative to the barycentric velocity of the system and (b) the motion of the system relative to the Hubble flow. Galaxies in clusters for which item (a) may cause significant distortions are generally excluded from analyses of geometrical structure. Although our knowledge concerning motions of systems of galaxies relative to the Hubble flow is still in an early state (Section 2.1.4), what we do know indicates that their effects can be neglected in estimating at least some broad features in the large-scale geometry of contiguous shells (42, 56, 57). Nevertheless, in detailed analyses, Fujimoto (79) and Kaiser (100b) demonstrated with several specific examples that the neglect of effects of peculiar motions relative to the Hubble flow can introduce some major geometrical distortions; for example, the infall motion caused by the self-gravity of a supercluster translates into a significantly sharpened contiguous shell, as estimated from uncorrected maps in velocity space (see Figure 4a).