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

2.1.4. KINEMATICS 0F CONTIGUOUS SHELLS

Previous reviews include Davis & Peebles (54) on evidence for local anisotropy of the Hubble flow, Mould (121) on the motion of the Local Group relative to the nearest clusters of galaxies, and Andersen (6) on recent indications of general streaming of galaxies within 100 Mpc.

Studies of galaxy samples within the Local Supercluster demonstrate that (a) the distance from the observed redshift by means of the Hubble relation and (b) the distance derived by a direct method that applies specific intrinsic structural properties of galaxies (e.g. the Tully-Fisher relation between H I velocity width and absolute luminosity of spiral galaxies, the Faber-Jackson relation between nuclear velocity dispersion and absolute luminosity of elliptical galaxies, and the approximately constant absolute luminosity of Sc I galaxies) display different systematic variations across the sky consistent with an infall velocity 300 km s^-1 of the Local Group toward the Virgo cluster (54).

Similar studies of samples of Sc and elliptical galaxies beyond the Local Supercluster and within a volume of radius 100 Mpc have led to the detection (in the frame defined by the galaxy sample) of a velocity component of the Local Group in addition to infall velocity (cf. 2, 6, 50a, 113b, and references therein). But the derived values are very uncertain, and systematic effects may be insufficiently understood. Therefore, the relation of this motion to that derived from the anisotropy of the 2.75K cosmic microwave background is currently unclear. The 0.1% cosine dependence (on sky angle) of the cosmic blackbody radiation is most simply interpreted to indicate that our galaxy (also the Local Group) is moving (in the universal frame defined by the cosmic blackbody radiation) at a velocity of 600 ± 50 km s^-1 toward a point in the constellation Hydra (212a). A recent study of an ensemble of galaxy clusters with distances between 80 and 200 Mpc indicates that the motion of the Local Group is within 250 km s^-1 of that inferred from this microwave dipole anisotropy (1a, 121). Evidently, the velocity of the Local Group (in the universal frame defined by the cosmic blackbody radiation) contains a component in addition to its infall velocity (i.e. the component caused by the gravitational attraction of the Local Supercluster). This additional component may represent motion of the entire Local Supercluster. On the other hand, A. Yahil (private communication, 1987) reports that results from a study of the three-dimensional distribution of IRAS infrared galaxies (which are detectable into the obscuring layer of the Milky Way) indicate that the additional component represents transverse motion of the Local Group within the Local Supercluster or its very near proximity.

The detected infall velocity of the Local Group suggests that superclusters, i.e. the components of contiguous shells, expand internally at rates that are slower than the rate of universal expansion. It follows that the observed velocity of a local region on the perimeter of a void relative to its center (i.e. the velocity caused by its origin and dynamical evolution) contains (a) a positive component corresponding to the infall of galaxies within a supercluster and (b) an additional component corresponding to the cause of the difference between the 600 km s^-1 velocity relative to the microwave background and the infall velocity of the Local Group. An outward velocity component of a void perimeter of ~ 500 km s^-1 corresponds to a change in its characteristic length of ~ 20 Mpc in a Hubble time ~ H₀^-1.