Charlier's (1922) hierarchical universe assumes clustering on increasingly larger scales:
``N1 stars together a galaxy G1,
N2 galaxies form together a Galaxy of the second order G2
N3 Galaxies of the second order galaxies form together a Galaxy of the third order third order G3, a.s.f.''
Lundmark (1925) wrote:
``Our Stellar system and the system of spiral nebulae are constructed according to the conceptions expressed in the Lambert-Charlier cosmogony.''
So far observational evidence has been found for G2 (clusters of galaxies), G3 (superclusters of galaxies), and possibly G4 (super-superclusters).
Shapley (1934) used the terms cluster of galaxies and supergalaxy interchangeably (``Undoubtedly the most important of the clusters of galaxies now known is the supergalaxy in Virgo''). With his reference to the double supergalaxy in Hercules he also introduced higher order clustering.
The term `supergalaxy' assumes a new quality in the paper by de Vaucouleurs (1953). Here the expression clearly refers to the local supercluster, as is apparent from his figure [here Fig. 12] in which he also includes the southern supergalaxy. The local supercluster has recently been extended to dimensions of the order of hundreds of Megaparsec (Tully 1987). Dimensions of other superclusters, approaching the 103 Mpc regime, are reported (Ford et al. 1981).
Figure 12. Presentation of the local
It should be noted that the basic data used by de Vaucouleurs are redshifts obtained by Rubin (1951). Velocities, especially those indicating systematic deviations from the Hubble flow, have significantly contributed to the development of the concept of mass concentrations.
As late as 1959, Zwicky rejected the idea of second order clustering for galaxies. A chapter heading in his article ``Clusters of Galaxies'' reads ``Superclustering non-existent''.
Contrary to this view Abell (1958) had concluded from a detailed statistical analysis of his cluster catalogue that:
``An analysis of the distribution [of rich clusters of galaxies] yields evidence that suggests the existence of second-order clusters, that is, clusters of clusters of galaxies. A statistical test reveals no incompatibilities between the observed distribution and one of complete second-order clustering of galaxies.''
A much stronger statement was made in 1982 (Einasto et al.):
``Recent studies of the spatial distribution of galaxies and of clusters indicate that practically all clusters and a vast majority of galaxies are concentrated into superclusters. The space between superclusters has no rich clusters and very few galaxies. The whole structure is cellular, with cell walls formed from sheetlike superclusters (2) and the empty cell interiors being huge voids.''
A possible definition of superclusters was suggested by Oort (1983):
``The larger and most conspicuous of these agglomerations may contain several clusters, which explains why they have been given the name `superclusters'. In their longer dimensions, crossing times exceed the age of the Universe. They are thus unrelaxed. Unrelaxed appearance together with large size might be taken as a definition of superclusters.''
Other descriptions of large-scale distributions use the expressions
(Gott et al. 1986)
or Voronoi foam
(Icke and van de
2 These terms are not merely descriptive, they are based on definite dynamical theories, see Sect. 2.3.2. Back.