Galaxy cluster

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Galaxy cluster ACO 3341.
The most distant mature galaxy cluster,[1] taken with ESO's Very Large Telescope in Chile and the NAOJ’s Subaru telescope in Hawaii

Galaxy clusters are the largest known structures in the universe. They consist of hundreds of galaxies bound by gravity.[2] Galaxy clusters are much larger than galaxy groups, such as the Local Group. Clusters of galaxies should not be confused with either star clusters, which are inside galaxies, or with globular clusters, which usually orbit galaxies.

Notable galaxy clusters in the relatively nearby universe include the Virgo Cluster, Fornax 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's law).

Notable galaxy clusters in the distant, high-redshift universe include SPT-CL J0546-5345, the most massive galaxy cluster ever found in the early universe.

Basic properties[change | edit source]

Galaxy clusters typically have the following properties.

  • They contain 50 to 1,000 galaxies, hot X-ray emitting gas and large amounts of dark matter. Details are described in the "Composition" section.
  • The distribution of these three components is approximately the same in the cluster.
  • They have total masses of 1014 to 1015 solar masses.
  • They typically have a diameter from 2 to 10 megaparsecs.
  • The spread of velocities for the individual galaxies is about 800–1000 km/s.
  • The intracluster medium or ICM has gas between the galaxies with a temperature of 7-9 keV.

Composition[change | edit source]

There are three main components of a galaxy cluster. They are:

Name of the components Mass fraction Description
Galaxies 1% In optical observations only galaxies are visible
Gas between galaxies, inside the cluster 9% Plasma between the galaxies at high temperature – emit x-ray radiation
Dark matter 90% Most massive component, cannot be seen, inferred by gravitational interactions

References[change | edit source]