A galaxy cluster, or cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity with typical masses ranging from 1014-1015 solar masses. They are the largest known gravitationally bound structures in the universe and were believed to be the largest known structures in the universe until the 1980s, when superclusters were discovered. One of the key features of clusters is the intracluster medium (ICM). The ICM consists of heated gas between the galaxies and has a peak temperature between 2-15 keV that is dependent on the total mass of the cluster. Galaxy clusters should not be confused with star clusters, such as galactic clusters--also known as open clusters--which are structures of stars within galaxies, or with globular clusters, which typically orbit galaxies. Small aggregates of galaxies are referred to as galaxy groups rather than clusters of galaxies. The galaxy groups and clusters can themselves cluster together to form superclusters.
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. Notable galaxy clusters in the distant, high-redshift Universe include SPT-CL J0546-5345 and SPT-CL J2106-5844, the most massive galaxy clusters found in the early Universe. In the last few decades, they are also found to be relevant sites of particle acceleration, a feature that has been discovered by observing non-thermal diffuse radio emissions, such as radio halos and radio relics. Using the Chandra X-ray Observatory, structures such as cold fronts and shock waves have also been found in many galaxy clusters.
Galaxy clusters typically have the following properties:
|Name of the components||Mass fraction||Description|
|Galaxies||1%||In optical observations, only galaxies are visible|
|Intergalactic gas in intracluster medium||9%||Plasma between the galaxies at high temperature and emit x-ray radiation by thermal bremsstrahlung|
|Dark matter||90%||Most massive component but cannot be detected optically and is inferred through gravitational interactions|
|Virgo Cluster||The nearest massive galaxy cluster|
|Norma Cluster||The cluster at the heart of the Great Attractor|
|Bullet Cluster||A cluster merger with the first observed separation between dark matter and normal matter|
|This lists some of the most notable clusters; for more clusters, see the list article.|
Galaxy cluster WHL J24.3324-8.477.
Background galaxy has been gravitationally lensed by the intervening galaxy cluster.
The galaxy cluster Abell 2813 (also known as ACO 2813) image from the NASA/ESA Hubble Space Telescope
A Menagerie of Galaxies -- The galaxy cluster ACO S 295