Editing Galaxy groups and clusters (section)

== Observational methods ==
[[File:Galaxy Cluster LCDCS-0829.jpg|thumb|Galaxy Cluster LCDCS-0829 acting like a giant magnifying glass. This strange effect is called [[gravitational lensing]].]]

Clusters of galaxies have been found in [[astronomical catalog|surveys]] by a number of observational techniques and have been studied in detail using many methods:
* [[Visible-light astronomy|Optical]] or [[Infrared astronomy|infrared]]:  The individual galaxies of clusters can be studied through optical or infrared imaging and spectroscopy. Galaxy clusters are found by optical or infrared telescopes by searching for overdensities, and then confirmed by finding several galaxies at a similar [[redshift]]. Infrared searches are more useful for finding more distant (higher [[redshift]]) clusters.
* [[X-ray astronomy|X-ray]]:  The hot plasma emits X-rays that can be detected by [[X-ray]] [[telescope]]s. The cluster gas can be studied using both X-ray imaging and X-ray spectroscopy. Clusters are quite prominent in X-ray surveys and along with [[Active galactic nucleus|AGN]] are the brightest X-ray emitting extragalactic objects.
* [[Radio astronomy|Radio]]:  A number of [[radio halo|diffuse structures]] emitting at radio frequencies have been found in clusters. Groups of radio sources (that may include [[radio halo|diffuse structures]] or AGN) have been used as tracers of cluster location. At high [[redshift]] imaging around individual radio sources (in this case AGN) has been used to detect proto-clusters (clusters in the process of forming).
* [[Sunyaev-Zel'dovich effect]]:  The hot electrons in the intracluster medium scatter radiation from the [[cosmic microwave background]] through inverse [[Compton scattering]]. This produces a "shadow" in the observed [[cosmic microwave background]] at some radio frequencies.
* [[Gravitational lens]]ing:  Clusters of galaxies contain enough matter to distort the observed orientations of galaxies behind them. The observed distortions can be used to model the distribution of dark matter in the cluster.