A galaxy cluster or cluster of galaxies is a structure that consists of anywhere from hundreds to thousands of galaxies bound together by gravity. They are the largest known gravitationally bound structures in the universe and were 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 or ICM. The ICM consists of heated gas between the galaxies and has a temperature on the order of 7-9 keV. Galaxy clusters should not be confused with star clusters such as open clusters, which are structures of stars within galaxies, as well as globular clusters, which typically orbit galaxies. Small aggregates of galaxies are referred to as groups of galaxies rather than clusters of galaxies. The 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, shock waves, and minihalos have also been found in many galaxy clusters.
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 Mpc (see 1023 m for distance comparisons).
- The spread of velocities for the individual galaxies is about 800–1000 km/s.
There are three main components of a galaxy cluster. They are tabulated below:
|Name of the components||Mass fraction||Description|
|Galaxies||1%||In optical observations only galaxies are visible|
|Intergalactic gas in ICM||9%||Plasma between the galaxies at high temperature – emit x-ray radiation through thermal bremsstrahlung mechanism|
|Dark matter||90%||Most massive component, cannot be detected optically, inferred through gravitational interactions|
- "Hubble Pinpoints Furthest Protocluster of Galaxies Ever Seen". ESA/Hubble Press Release. Retrieved 13 January 2012.
- Kravtsov, A. V.; Borgani, S. (2012). "Formation of Galaxy Clusters". Annual Review of Astronomy and Astrophysics 50: 353. arXiv:1205.5556. Bibcode:2012ARA&A..50..353K. doi:10.1146/annurev-astro-081811-125502.
- Clavin, Whitney; Jenkins, Ann; Villard, Ray (7 January 2014). "NASA's Hubble and Spitzer Team up to Probe Faraway Galaxies". NASA. Retrieved 8 January 2014.
- "Magnifying the distant Universe". ESA/Hubble Picture of the Week. Retrieved 10 April 2014.