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 In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...

, 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 The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the univers ...

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 t ...

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 cluster An open cluster is a type of star cluster made of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way galaxy, an ...

s), 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 cl ...

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 Galaxy groups and clusters are the largest known gravitationally bound objects to have arisen thus far in the process of cosmic structure formation. They form the densest part of the large-scale structure of the Universe. In models for the grav ...

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 Her ...

, and the

Coma Cluster The Coma Cluster (Abell 1656) is a large cluster of galaxies that contains over 1,000 identified galaxies. Along with the Leo Cluster (Abell 1367), it is one of the two major clusters comprising the Coma Supercluster. It is located in and tak ...

. 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 The Chandra X-ray Observatory (CXO), previously known as the Advanced X-ray Astrophysics Facility (AXAF), is a Flagship-class space telescope launched aboard the during STS-93 by NASA on July 23, 1999. Chandra is sensitive to X-ray sources ...

, 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 An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

emitting gas and large amounts of

dark matter Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not ...

. 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 General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics ...

: 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 In physics and general relativity, gravitational redshift (known as Einstein shift in older literature) is the phenomenon that electromagnetic waves or photons travelling out of a gravitational well (seem to) lose energy. This loss of energy ...

. 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 Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not ...

that does not interact with matter.

Gravitational lensing

Galaxy clusters are also used for their strong gravitational potential as gravitational lenses 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 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 ...

cluster with the mass of two quadrillion Suns. File:Streaks and stripes Abell 2537.jpg, Abell 2537 is useful in probing cosmic phenomena like dark matter and dark energy. File:Cosmic RELICS Abell 1300.jpg, Abell 1300 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 (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 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. File:14-296-GalaxyClusters-PerseusVirgo-ChandraXRay-20141027.jpg,

Turbulence In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows in parallel layers, with no disruption between ...

may prevent galaxy clusters from cooling ; illustrated:

Perseus Cluster The Perseus cluster (Abell 426) is a cluster of galaxies in the constellation Perseus. It has a recession speed of 5,366 km/ s and a diameter of 863′. It is one of the most massive objects in the known universe, containing thousands of ...

and Virgo Cluster ( Chandra X-ray). File:Color image of galaxy cluster MCS J0416.1–2403.jpg, MACS0416.1-2403 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 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 vers ...

File:A_Menagerie_of_Galaxies.jpg, A Menagerie of Galaxies — The galaxy cluster ACO S 295 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, with a redshift of 2.156, which is 10.6 billion light years from the Milky Way. It has been imaged by the Hubble Space Telescope on 12 Oct ...

(artist's concept).

References

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