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:
There are three main components of a galaxy cluster. They are tabulated below:
Galaxy clusters are categorized as type I, II, or III based on morphology.
Galaxy cluster SPT-CL J0615-5746.
Galaxy cluster RXC J0232.2-4420.
Galaxy cluster RXC J0032.1+1808 as part of the RELICS program.
Massive galaxy cluster PSZ2 G138.61-10.84 is about six billion light-years away.
HAWK-I and Hubble explore RCS2 J2327 cluster with the mass of two quadrillion Suns.
Abell 2537 is useful in probing cosmic phenomena like dark matter and dark energy.
Abell 1300 acts like a lens, bending the very fabric of space around it.
Galaxy cluster WHL J24.3324-8.477.
Background galaxy has been gravitationally lensed by the intervening galaxy cluster.
"Smiley" image – galaxy cluster (SDSS J1038+4849) & gravitational lensing (an Einstein ring) (HST).
Galaxy cluster SpARCS1049 taken by Spitzer and the Hubble Space Telescope.
Galaxy cluster MOO J1142+1527 discovered by the MaDCoWS survey
Abell 2744 galaxy cluster (HST).
Magnifying the distant universe through MACS J0454.1-0300.
Turbulence may prevent galaxy clusters from cooling ; illustrated: Perseus Cluster and Virgo Cluster (Chandra X-ray).
MACS0416.1-2403 imaged by the HST
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
Cosmic Lens Flare
Video: Formation of galaxy cluster MRC 1138-262 (artist's concept).