A galaxy cluster, or a cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of

galaxies A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...

that are bound together by gravity, with typical masses ranging from 10¹⁴ to 10¹⁵

solar mass The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass ...

es. They are the second-largest known gravitationally bound structures in the universe after galaxy filaments and were believed to be the largest known structures in the universe until the 1980s, when

supercluster A supercluster is a large group of smaller galaxy clusters or galaxy groups; they are among the largest known structures in the universe. The Milky Way is part of the Local Group galaxy group (which contains more than 54 galaxies), which in turn ...

s 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 ''galactic clusters'' (also known as open clusters), which are

star cluster Star clusters are large groups of stars. Two main types of star clusters can be distinguished: globular clusters are tight groups of ten thousand to millions of old stars which are gravitationally bound, while open clusters are more loosely clust ...

s ''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 The Hercules Cluster ( Abell 2151) is a cluster of about 200 galaxies some 500 million light-years distant in the constellation Hercules. It is rich in spiral galaxies and shows many interacting galaxies. The cluster is part of the larger Hercul ...

, 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 SPT-CL J0546-5345 is one of the most massive galaxy clusters ever found in the early universe. It is thought to be 7 billion light years away. It was discovered at the South Pole Telescope in 2008 by the Sunyaev-Zel'dovich-Effect. The cl ...

and

SPT-CL J2106-5844 SPT-CLJ2106-5844 is a galaxy cluster located 7.5 billion light years from Earth. It was discovered by scientists from the South Pole Telescope Collaboration, using the South Pole Telescope The South Pole Telescope (SPT) is a diameter telescope ...

, 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 halo Radio halos are large-scale sources of diffuse radio emission found in the center of some, but not all, galaxy clusters.Feretti, L., and G. Swarup. "The Universe at Low Radio Frequencies." Proceedings of IAU Symposium. Vol. 199. 2002. There are t ...

s and

radio relics Radio Relics are diffuse synchrotron radio sources found in the peripheral regions of galaxy clusters. As in the case of radio halos, they do not have any obvious galaxy counterpart, but their shapes are much more elongated and irregular compar ...

. Using the Chandra X-ray Observatory, structures such as cold fronts and shock waves have also been found in many galaxy clusters.

Basic properties

Galaxy clusters typically have the following properties: * They contain 100 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 the 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 1 to 5 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:

Classification

Galaxy clusters are categorized as type I, II, or III based on morphology.

Galaxy clusters as measuring instruments

Gravitational redshift

Galaxy clusters have been used by Radek Wojtak from the Niels Bohr Institute at the University of Copenhagen to test predictions of general relativity: energy loss from light escaping a gravitational field. Photons emitted from the center of a galaxy cluster should lose more energy than photons coming from the edge of the cluster because gravity is stronger in the center. Light emitted from the center of a cluster has a longer wavelength than light coming from the edge. This effect is known as gravitational redshift. Using the data collected from 8000 galaxy clusters, Wojtak was able to study the properties of gravitational redshift for the distribution of galaxies in clusters. He found that the light from the clusters was redshifted in proportion to the distance from the center of the cluster as predicted by general relativity. The result also strongly supports the Lambda-Cold Dark Matter model of the Universe, according to which most of the cosmos is made up of Dark Matter that does not interact with matter.

Gravitational lensing

Galaxy clusters are also used for their strong gravitational potential as

gravitational lenses A gravitational lens is a distribution of matter (such as a cluster of galaxies) between a distant light source and an observer that is capable of bending the light from the source as the light travels toward the observer. This effect is known ...

to boost the reach of their telescopes. The gravitational distortion of space-time occurs near massive galaxy clusters and bends the path of photons to create a cosmic magnifying glass. This can be done with photons of any wavelength from the optical to the X-ray band. The latter is more difficult, because galaxy clusters emit a lot of X-rays. However, X-ray emission may still be detected when combining X-ray data to optical data. One particular case is the use of the Phoenix galaxy cluster to observe a dwarf galaxy in its early high energy stages of star formation.

List

Gallery

Images

File:Distant_and_ancient_SPT0615-JD.jpg, Galaxy cluster SPT-CL J0615-5746. File:Strings of homeless stars RXC J0232.2-4420.jpg, Galaxy cluster RXC J0232.2-4420. File:From toddlers to babies RXC J0032.1+1808.jpg, Galaxy cluster RXC J0032.1+1808 as part of th
RELICS
program. File:Approaching the Universe's origins PSZ2 G138.61-10.84.jpg, Massive galaxy cluster PSZ2 G138.61-10.84 is about six billion light-years away. File:HAWK-I and Hubble Explore a Cluster with the Mass of two Quadrillion Suns.jpg, HAWK-I and

Hubble The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...

explore

RCS2 J2327 RCS2 J2327 (also identified as RCS2 J2327-0204) is an extremely massive galaxy cluster. It is located approximately 6.4 billion light-years away in the constellation of Pisces, thus making it one of the farthest clusters away from Earth. Recent s ...

cluster with the mass of two quadrillion Suns. File:Streaks and stripes Abell 2537.jpg,

Abell 2537 Abell may refer to: People *Abell (surname) *George O. Abell, of the astronomical catalogues fame Places ;United States * Abell, Maryland, a location in St. Mary's County, Maryland * Abell, a neighborhood in Baltimore, Maryland * Abells Corners, ...

is useful in probing cosmic phenomena like dark matter and dark energy. File:Cosmic RELICS Abell 1300.jpg,

Abell 1300 Abell may refer to: People *Abell (surname) *George O. Abell, of the astronomical catalogues fame Places ;United States * Abell, Maryland, a location in St. Mary's County, Maryland * Abell, Baltimore, Abell, a neighborhood in Baltimore, Maryland ...

acts like a lens, bending the very fabric of space around it. File:Cosmic archaeology WHL J24.3324-8.477.jpg, Galaxy cluster WHL J24.3324-8.477. File:Hubble pushed beyond limits to spot clumps of new stars in distant galaxy.jpg, Background galaxy has been gravitationally lensed by the intervening galaxy cluster. File:HST-Smiling-GalaxyClusterSDSS-J1038+4849-20150210.jpg, " Smiley" image – galaxy cluster (SDSS J1038+4849) &

gravitational lensing A gravitational lens is a distribution of matter (such as a galaxy cluster, cluster of galaxies) between a distant light source and an observer that is capable of bending the light from the source as the light travels toward the observer. This ...

(an Einstein ring) ( HST). File:Image of the galaxy cluster SpARCS1049.jpg, Galaxy cluster SpARCS1049 taken by Spitzer and the Hubble Space Telescope. File:PIA20052-GalaxyCluster-MOO-J1142+1527-20151103.jpg, Galaxy cluster

MOO J1142+1527 A MOO ("MUD, object-oriented") is a text-based online virtual reality system to which multiple users (players) are connected at the same time. The term MOO is used in two distinct, but related, senses. One is to refer to those programs descend ...

discovered by the MaDCoWS survey File:Heic1401a-Abell2744-20140107.jpg, Abell 2744 galaxy cluster ( HST). File:Magnifying the distant Universe.jpg, Magnifying the distant universe through

MACS J0454.1-0300 Macs or MACS may refer to: *a Mac computer, a nickname for the Macintosh brand * "macs" (short story), by Terry Bisson *Magellanic Catalogue of Stars, a catalogue of positions for stars covering large areas around the Large and Small Magellanic Clo ...

. File:14-296-GalaxyClusters-PerseusVirgo-ChandraXRay-20141027.jpg, Turbulence may prevent galaxy clusters from cooling ; illustrated: Perseus Cluster and Virgo Cluster ( Chandra X-ray). File:Color image of galaxy cluster MCS J0416.1–2403.jpg,

MACS0416.1-2403 MACS J0416.1-2403 is a cluster of galaxies at a redshift of z=0.397 with a mass 160 trillion times the mass of the Sun inside . Its mass extends out to a radius of and was measured as 1.15 × 1015 solar masses. The system was discovered in images ...

imaged by the HST File:Light_Bends_from_the_Beyond.jpg, The galaxy cluster Abell 2813 (also known as ACO 2813) image from the NASA/ESA Hubble Space Telescope File:A_Menagerie_of_Galaxies.jpg, A Menagerie of Galaxies — The galaxy cluster

ACO S 295 ACO, AcO, or Aco may refer to: Organizations * Accountable care organization, healthcare organization characterized by a specific payment and care delivery model * Aco Records, a British 1920s record label * ACO, C.A., a Venezuelan holding compan ...

File:Cosmic_Lens_Flare.jpg, Cosmic Lens Flare File:Hubble spots three images of a distant supernova.jpg, Hubble spots three images of a distant supernova

Videos

File:Artist’s impression of a protocluster forming in the early Universe.ogv, Video: Formation of galaxy cluster

MRC 1138-262 The Spiderweb Galaxy (PGC 2826829, MRC 1138-262) is an irregular galaxy located in the Hydra (constellation), Hydra constellation, with a redshift of 2.156, which is 10.6 billion light years from the Milky Way. It has been imaged by the Hubble Spa ...

(artist's concept).

References

{{Portal bar, Astronomy, Stars, Spaceflight, Outer space, Solar System Cluster Articles containing video clips