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Galaxy groups and clusters are the largest gravitationally-bound objects. They form the densest part of the large scale structure of the Universe. In models for the gravitational formation of structure with cold dark matter, the smallest structures collapse first and eventually build the largest structures, clusters of galaxies. Clusters are then formed relatively recently between 10 billion years ago and now. Groups and clusters may contain from ten to thousands of galaxies. The clusters themselves are often associated with larger groups called superclusters.
Groups of galaxies Groups of galaxies are the smallest aggregates of galaxies. They typically contain fewer than 50 galaxies in a diameter of 1 to 2 Megaparsecs (Mpc) (see 1 E22 m for distance comparisons). Their mass are approximately 1013 solar masses. The spread of velocities for the individual galaxies is about 150 km/s. However this definition should be used as a guide only, as larger and more massive galaxy systems are sometimes classified as galaxy groups. The group which contains our own galaxy, the Milky Way, is called the Local Group, and contains more than 40 galaxies. Clusters of galaxies Clusters are larger than groups, although there is no sharp dividing line between a group and a cluster. When observed visually, clusters appear to be collections of galaxies held together by mutual gravitational attraction. However, their velocities are too large for them to remain gravitationally-bound by their mutual attractions, implying the presence of either an additional invisible mass component, or an additional attractive force besides gravity. X-ray studies have revealed the presence of large amounts of intergalactic gas known as the intracluster medium. This gas is very hot, between 107K and 108K, and hence emits X-rays in the form of bremsstrahlung and atomic line emission. The total mass of the gas is greater than that of the galaxies by roughly a factor of two. However this is still not enough mass to keep the galaxies in the cluster. Since this gas is in approximate hydrostatic equilibrium with the overall cluster gravitational field, the total mass distribution can be determined. It turns out the total mass deduced from this measurement is approximately six times larger than the mass of the galaxies or the hot gas. The missing component is known as dark matter and its nature is unknown. In a typical cluster perhaps only 5% of the total mass is in the form of galaxies, maybe 10% in the form of hot X-ray emitting gas and the remainder is dark matter. Clusters typically have the following properties. Notable galaxy clusters in the relatively nearby universe include the Virgo cluster, Hercules Cluster, and the Coma Cluster. A very large aggregation of galaxies known as the Great Attractor, dominated by the Norma cluster, is massive enough to affect the local expansion of the universe (Hubble flow). Note: clusters of galaxies should not be confused with star clusters such as galactic clusters and open clusters, which are structures within galaxies, as well as globular clusters, which typically orbit galaxies. Superclusters Main article: Supercluster Groups, clusters and some isolated galaxies form even larger structures, the superclusters. At the very largest scales of the visible universe, matter is gathered into filaments and walls surrounding vast voids. This structure resembles a foam. Observational methods Clusters of galaxies have been found in surveys by a number of observational techniques and have been studied in detail using many methods: Instruments and surveys used for finding clusters of galaxies See also | ||||||||
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