Galaxy cluster - AbsoluteAstronomy.com

A galaxy cluster is a compact cluster of galaxies. Basic difference between a galaxy group

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...

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 cluster is the Intracluster medium

Intracluster medium

In astronomy, the intracluster medium is the superheated plasma present at the center of a galaxy cluster. This is gas heated to temperatures of between roughly 10 and 100 megakelvins and consisting mainly of ionised hydrogen and helium, containing most of the baryonic material in the cluster...

or ICM. ICM is constituted by gas between the galaxies and has temperature on the order of 7-9 keV. Clusters of galaxies should not be confused with star cluster

Star cluster

Star clusters or star clouds are groups of stars. Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds of thousands of very old stars which are gravitationally bound, while open clusters, more loosely clustered groups of stars, generally contain less than...

s such as galactic clusters and open cluster

Open cluster

An open cluster is a group 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, and many more are thought to exist...

s, which are structures within galaxies, as well as globular cluster

Globular cluster

A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The name of this category of star cluster is...

s, which typically orbit galaxies.
Notable galaxy clusters in the relatively nearby universe include the Virgo cluster

Virgo Cluster

The Virgo Cluster is a cluster of galaxies whose center is 53.8 ± 0.3 Mly away in the constellation Virgo. Comprising approximately 1300 member galaxies, the cluster forms the heart of the larger Local Supercluster, of which the Local Group is an outlying member...

, Fornax Cluster

Fornax Cluster

At a distance of approximately 62.0 Mly , the Fornax Cluster is the second richest cluster of galaxies within 100 million light-years, although it is much smaller than the Virgo Cluster. It lies primarily in the constellation Fornax, and may be associated with the nearby Eridanus Group...

, Hercules Cluster

Hercules Cluster

The Hercules Cluster is a cluster of about 100 galaxies some 650 million light-years distant in the constellation Hercules. It is rich in spiral galaxies and shows many interacting galaxies...

, and the Coma Cluster

Coma cluster of galaxies

The Coma Cluster is a large cluster of galaxies that contains over 1,000 identified galaxies. Along with the Leo Cluster , it is one of the two major clusters comprising the Coma Supercluster. It is located in and takes its name from the constellation Coma Berenices.The cluster's mean distance...

. A very large aggregation of galaxies known as the Great Attractor

Great Attractor

The Great Attractor is a gravity anomaly in intergalactic space within the range of the Centaurus Supercluster that reveals the existence of a localised concentration of mass equivalent to tens of thousands of Milky Ways, observable by its effect on the motion of galaxies and their associated...

, dominated by the Norma cluster

Norma cluster

The Norma Cluster is a rich cluster of galaxies located near the center of the Great Attractor. Although it is both nearby and bright, it is difficult to observe because it is located in the Zone of Avoidance, a region near the plane of the Milky Way. Consequently, the cluster is severely...

, 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

SPT-CL J0546-5345

SPT-CL J0546-5345is 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 cluster has a redshift of z=1.067...

, 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 halo

Radio halo

Radio halos are large-scale areas of radio emission found in clusters of galaxies. They do not have an obvious galaxy counterpart, as opposed, for example, to radio galaxies which have AGN counterparts...

s and radio relics

Radio relics

Radio Relics are diffuse synchrotron radio emission found in the peripheral regions of galaxy clusters. Similar to the case of radio halos, they do not have any obvious galaxy counterpart, but their shapes are much more elongated and irregular compared to those of radio halos...

. With Chandra X-ray Observatory

Chandra X-ray Observatory

The Chandra X-ray Observatory is a satellite launched on STS-93 by NASA on July 23, 1999. It was named in honor of Indian-American physicist Subrahmanyan Chandrasekhar who is known for determining the maximum mass for white dwarfs. "Chandra" also means "moon" or "luminous" in Sanskrit.Chandra...

structures like cold front

Cold front

A cold front is defined as the leading edge of a cooler mass of air, replacing a warmer mass of air.-Development of cold front:The cooler and denser air wedges under the less-dense warmer air, lifting it...

, shock front

Shock Front

Shock Front is the first full length album by Converter, released November 22, 1999 . The album is released in two editions, the first featuring a special metal plate packaging , the second featuring a regular cardboard booklet format .-Track listing:# "Conqueror"–8:05# "Shock Front"–7:15#...

, minihalo

Radio halo

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
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...

. 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
1 E23 m
To help compare different orders of magnitude, this page lists distances starting at 100 Zm .Distances shorter than 100 Zm* 140 Zm — 15 million light years — Distance to Centaurus A galaxy...

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 Bremsstrahlung Bremsstrahlung is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into a photon because energy is conserved. The term is... mechanism.
Dark matter	90%	Most massive component, cannot be detected optically, inferred through gravitational interactions.

Basic properties

Composition

See also