Superclusters of Galaxies

Structure is observed on larger scales than that of clusters. The next larger scale corresponds to superclusters, which can have extents of order 100 Mpc (about 325 million light years). All known rich clusters are parts of larger superclusters, suggesting that perhaps all galaxies belong to one supercluster or another, but this is not yet certain.
An Example: the Local Supercluster
Our own galaxy and its Local Group belong to a supercluster called the Local Supercluster. It similar in shape to a flattened ellipse (pancake), with the Virgo Cluster near its center and the Local Group near one end; its extent in the longest direction is about 40-50 Mpc. The following diagram shows a projection in right ascension and declination of galaxies within the Local Supercluster, as viewed from Earth.

The plot is centered on right ascension 6 hours and declination zero degrees, and the diagonal lines across the center mark the zone of avoidance where distant galaxies are obscured from our view by dust in the galactic plane. The larger concentration of galaxies on the left side of the diagram than the right side is because we are out near an edge of the Local Supercluster. As a consequence, when we look toward its center (on the left side, roughly in Virgo) we see many more galaxies than when we look in the opposite direction in the sky (on the right side).

Technically Speaking: Peculiar Velocity and the Local Supercluster

As we shall see, the Virgo Cluster at a distance of approximately 18 Mpc from us should, by the Hubble Law, be receding from us at a velocity of about 1100 km/s with the general expansion of the Universe. However, the measured recessional velocity of the Virgo Cluster is approximately 170 km/s less than this. This difference presumably is due to the higher than average gravitational attraction felt between the Local Group and the rest of the Local Supercluster.

Motion within the Local Supercluster
Shortly we shall discuss the expansion of the Universe. This general expansion, which increases the distances between galaxies steadily with time, is called the Hubble flow and is governed by the Hubble law. Deviation of the velocity of a galaxy from the overall Hubble flow is termed its peculiar velocity. By examining the peculiar velocities of clusters and superclusters, we can obtain estimates of local mass concentrations that may be responsible for causing the deviation from the Hubble flow (see the adjacent box).
Mass of the Local Supercluster
A mass of 1015 solar masses may be estimated for the Local Supercluster, based on peculiar velocities such as those described in the above right box. The corresponding mass to light ratio is about 570, expressed in units of the ratio of the solar mass to the solar luminosity. This indicates the presence of large amounts of dark matter in the Local Supercluster.