|Galaxy Cluster Cloud|
|Canes Venatici Cloud|
A supercluster is a group of galaxy clusters that seem associated with each other. Usually it is decided which galaxy clusters together form a supercluster by looking at the distances between the galaxy clusters. This method works well if the superclusters are clearly separated from each other, but if galaxy clusters can be found at all distances between the centers of two neighboring superclusters (as is often the case) then it may not be easy to decide whether a particular galaxy cluster about halfway between the supercluster centers belongs to the one or to the other supercluster.
Distances to galaxies and galaxy clusters are much more difficult to determine than the directions in which those objects are, so distances between such objects are usually much more accurate perpendicular to the radial direction than they are in the radial direction. This makes determination of which galaxy clusters belong to which superclusters more difficult, too.
A definition for superclusters that has less ambiguity would be a group of galaxy clusters that are bound together by gravity, just like the Sun, all the planets, and all other objects in the solar system are bound together by gravity. Such information is currently not known for most galaxy clusters, and it seems likely that not all galaxy clusters are bound by gravity to any one supercluster, so this definition also has its problems.
Our galaxy lies in the Canes Venatici cloud of galaxies on the outskirts of the Local Supercluster, of which the Virgo galaxy cluster is the central part . The Virgo galaxy cluster is the only rich galaxy cluster in the Local Supercluster, and none of the listed galaxy clouds has its center further from the Virgo cluster than our own.
The major components (galaxy clouds) of the Local Supercluster are listed in the following table. "Name" is the common name of the cloud (an NGC number indicates the most important member of the cloud) , "Dist" the distance of its center to us, "Size" its greatest extent, "Volume" its approximate volume, "N" the number of bright galaxies in the cloud (absolute magnitude brighter than −18.7, which is about the estimated brightness of our own Galaxy), and "Dens" the number of bright galaxies per unit volume. All distances are measured in Mpc. The number of fainter galaxies in these galaxy clouds is likely much greater than the number of bright galaxies.
Table 1: Local Supercluster of Galaxy Clusters
|Virgo II (S)||16.3||14.9||560||55||0.10|
|Crater (NGC 3672)||21.0||11.4||340||25||0.07|
|Leo Minor (NGC 2841)||7.2||10.6||170||11||0.06|
|Draco (NGC 5907)||13.5||11.4||170||6||0.04|
|Antlia (NGC 2997)||10.6||6.5||110||5||0.05|
The Local Supercluster appears made of two major structures: a flattened disk with a thickness of about 1.4 Mpc which contains about 60% of the bright galaxies, and a roughly spherical halo which contains the remaining 40% of the bright galaxies in a small number of clouds.
One recent survey of superclusters up to redshift z = 0.1 (about 400 Mpc) finds 130 superclusters . The ones that are closer to us than 140 Mpc, those that contain at least 10 galaxy clusters, and those that have a proper name in the survey are listed below. In the table, "Id" means the identification in the survey, "Name" the proper name of the supercluster (usually made of the one or two constellations that the superclusters appears in, but the "Shapley" supercluster is named for its discoverer), "Dist" the distance of the center of the supercluster to us, in Mpc, and "Members" the number of rich galaxy clusters that are members of the supercluster.
Table 2: Nearby Superclusters of Galaxy Clusters
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Last updated: 2016−01−28