Galaxy cluster
A galaxy cluster is a compact cluster of galaxies. The primary difference between a galaxy group and a galaxy cluster is that there are many more galaxies in a cluster than in a group. Also, galaxies in a cluster are more compact and have higher velocity dispersion. One of the key features of clusters is the Intracluster medium or ICM. ICM is constituted by gas between the galaxies and has a temperature on the order of 7-9 keV. 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.
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 flow). 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. In the last few decades, they are also found to be relevant sites of particle acceleration, a feature which has been discovered by the observing non-thermal diffuse radio emissions as radio halos and radio relics. With Chandra X-ray Observatory structures like cold front, shock front, minihalo have also been found in many galaxy clusters.
Basic properties
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.
Composition
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. |