| Annu. Rev. Astron. Astrophys. 1988. 26:
245-294 Copyright © 1988 by . 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.