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. 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 large concentration of galaxies on the left side of the following diagram represents part of the Local Supercluster (see this explanation).

The Local Supercluster (Source and Explanation)

Other nearby superclusters include Perseus-Pisces at a distance of about 70 Mpc and Hydra-Centaurus, which is approximately 45 Mpc distant (with the constellations in the names giving the approximate location on the celestial sphere; thus, Perseus-Pisces lies partially in the constellation Perseus and partially in Pisces, as viewed from the Earth).

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. Deviations of the velocity of a galaxy from the overall Hubble flow is termed the 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.

Mass of the Local Supercluster

As we shall see, the Virgo Cluster at a distance of approximately 16 Mpc from us should, by the Hubble Law, be receding from us at a velocity of about 1100 km/s. However, the measured recessional velocity of the Virgo Cluster is approximately 170 km/s less than this. This difference, which is termed the Virgocentric peculiar velocity of the Local Group, presumably is due to the higher than average gravitational attraction felt between the Local Group and the rest of the Local Supercluster.

This may be used to estimate a mass of 1015 solar masses for the Local Supercluster. The corresponding mass to light ratio for the Local Supercluster is about 570, expressed in units of the ratio of the solar mass to the solar luminosity, indicating the presence of large amounts of dark matter in the Local Supercluster.

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