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.