5.1. The Evidence for the Local Supercluster
A local irregularity in the distribution of nearby groups and clusters
of galaxies has
been suspected for several decades. From his analysis of the
distribution of double and multiple galaxies,
Holmberg (1937)
inferred the existence of a metagalactic cloud with
a diameter, according to the distance scale used in this review, between
90 and 150 Mpc,
and with a center lying in the general direction of the north galactic
pole at a distance
of about 18 Mpc. Holmberg's conclusion was confirmed qualitatively by
Reiz's (1941)
study of the distribution in direction and magnitude of 4000 galaxies in
the northern
galactic polar cap. The idea of a local supercluster of galaxies was
revived by
de Vaucouleurs (1953,
1956,
1958),
who described it in considerable detail. According to
de Vaucouleurs, the supercluster has a diameter (with H = 50 km
s-1 Mpc-1) of about
75 Mpc, and it contains, in addition to the Local Group, the
Virgo cluster, the Ursa Major
cloud, and numerous smaller groups and clusters. He finds the system to
be flattened, so
that the bright galaxies are seen in the sky highly concentrated toward
a great circle
(the "supergalactic equator") with its pole at
lII = 47°,
bII = 5°. He believes the center
of the system to lie within or near the Virgo cluster. The flattening
suggests rotation;
from his analysis of the radial velocities of bright galaxies, de
Vaucouleurs presents
interesting evidence for differential rotation, and derives about 500 km
s-1 for the
rotational velocity of the Galaxy about the center of the system. From
this rotational velocity,
he derives a total mass for the Local Supercluster of the order of
1015
. On the other
hand, the present or "instantaneous" period of revolution of the Galaxy
is about
2 × 1011 years; and even though de Vaucouleurs believes the
system to be expanding slowly,
it can hardly have completed even one rotation unless it was formed at a
very early
epoch in the expansion of the Universe, when its mean density was orders
of magnitude
higher than at present. Quite possibly, therefore, the apparent
flattening of the supercluster may have nothing to do with its presumed
rotation.
The dynamical properties of the Local Supercluster may not be well established, but further evidence for its reality as a geometrical entity is provided by an independent investigation by Carpenter. Carpenter (1961; Abell 1961) studied the distribution in magnitude and direction of galaxies brighter than mpg = 16 in a large region of the north galactic hemisphere from the Palomar Sky Survey prints. At magnitudes brighter than 13.5, he finds a highly significant concentration of galaxies along a 90° sector of an 18° strip along de Vaucouleurs's "supergalactic equator." In the next interval of 1 mag, the number of galaxies drops off very rapidly compared with expectations for a uniform galaxy distribution in depth. For mpg > 14.5, however, the logarithm of the number of galaxies brighter than mpg increases as 0.6mpg, which would be expected if most of these galaxies are remote ones, beyond the limits of the supercluster. Carpenter finds a similar result for galaxies in adjacent 18° strips saddling the strip along the supergalactic equator, except that the total number of bright galaxies in these zones is less than in the central strip.