Information about Galaxy Groups And Clusters

Galaxy groups and clusters are the largest gravitationally-bound objects. They form the densest part of the large scale structure of the Universe. In models for the gravitational formation of structure with cold dark matter, the smallest structures collapse first and eventually build the largest structures, clusters of galaxies. Clusters are then formed relatively recently between 10 billion years ago and now. Groups and clusters may contain from ten to thousands of galaxies. The clusters themselves are often associated with larger groups called superclusters.

Groups of galaxies

Groups of galaxies are the smallest aggregates of galaxies. They typically contain fewer than 50 galaxies in a diameter of 1 to 2 megaparsecs (Mpc) (see 1 E22 m for distance comparisons). Their mass is approximately 10¹³ solar masses. The spread of velocities for the individual galaxies is about 150 km/s. However, this definition should be used as a guide only, as larger and more massive galaxy systems are sometimes classified as galaxy groups.

The group which contains our own galaxy, the Milky Way, is called the Local Group, and contains more than 40 galaxies.

Clusters of galaxies

Clusters are larger than groups, although there is no sharp dividing line between a group and a cluster. When observed visually, clusters appear to be collections of galaxies held together by mutual gravitational attraction. However, their velocities are too large for them to remain gravitationally bound by their mutual attractions, implying the presence of either an additional invisible mass component, or an additional attractive force besides gravity. X-ray studies have revealed the presence of large amounts of intergalactic gas known as the intracluster medium. This gas is very hot, between 10⁷K and 10⁸K, and hence emits X-rays in the form of bremsstrahlung and atomic line emission. The total mass of the gas is greater than that of the galaxies by roughly a factor of two. However this is still not enough mass to keep the galaxies in the cluster. Since this gas is in approximate hydrostatic equilibrium with the overall cluster gravitational field, the total mass distribution can be determined. It turns out the total mass deduced from this measurement is approximately six times larger than the mass of the galaxies or the hot gas. The missing component is known as dark matter and its nature is unknown. In a typical cluster perhaps only 5% of the total mass is in the form of galaxies, maybe 10% in the form of hot X-ray emitting gas and the remainder is dark matter.

Clusters typically have the following properties.

• They contain 50 to 1000 galaxies, hot X-ray emitting gas and large amounts of dark matter
• 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 1 E23 m for distance comparisons).
• The spread of velocities for the individual galaxies is about 800-1000 km/s.

Notable galaxy clusters in the relatively nearby universe include the Virgo 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 (Hubble flow).

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 Halos and Radio Relics.

Note: clusters of galaxies should not be confused with star clusters such as galactic clusters and open clusters, which are structures within galaxies, as well as globular clusters, which typically orbit galaxies.

Superclusters

Main article: Supercluster

Groups, clusters and some isolated galaxies form even larger structures, the superclusters. At the very largest scales of the visible universe, matter is gathered into filaments and walls surrounding vast voids. This structure resembles a foam.

Observational methods

Clusters of galaxies have been found in surveys by a number of observational techniques and have been studied in detail using many methods:

• Optical or 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: The hot plasma emits X-rays which can be detected by X-ray telescopes. 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 AGN are the brightest X-ray emitting extragalactic objects.
• Radio: A number of diffuse structures emitting at radio frequencies have been found in clusters. Groups of radio sources (which may include 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 Lensing: 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.

Temperature, Density, and "Entropy"

Clusters of galaxies are the most recent and most massive objects to have arisen in the hierarchical structure formation of the Universe and the study of clusters tells one about the way galaxies form and evolve. Clusters have two important properties: their masses are large enough to retain any energetic gas ejected from member galaxies and the thermal energy of the gas within the cluster is observable within the X-Ray bandpass. The observed state of gas within a cluster is determined by a combination of shock heating during accretion, radiative cooling, and thermal feedback triggered by that cooling. The density, temperature, and substructure of the intracluster X-Ray gas therefore represents the entire thermal history of cluster formation. To better understand this thermal history one needs to study the entropy of the gas because entropy is the quantity most directly changed by increasing or decreasing the thermal energy of intracluster gas.

In astrophysics, what is referred to as "entropy" is actually the adiabatic constant derived as follows.

Using the first law of thermodynamics for a quasi-static, infinitesimal process for a hydrostatic system

For an ideal gas in this special case, the internal energy, U, is only a function of T; therefore the partial derivative of heat capacity with respect to T is identically the same as the full derivative, yielding through some manipulation

Further manipulation using the differential version of the ideal gas law, the previous equation, and assuming constant pressure, one finds

For an adiabatic process and recalling , one finds

One can solve this simple differential equation to find

This equation is known as an expression for the adiabatic constant, K, also called the adiabat. From the ideal gas equation one also knows

where is Boltzmann's constant. Substituting this into the above equation along with and for an ideal monoatomic gas one finds

where is the mean molecular weight of the gas or plasma; and is the mass of the Hydrogen atom, which is extremely close to the mass of the proton, , the quantity more often used in astrophysical theory of galaxy clusters. This is what astrophysicists refer to as "entropy" and has units of [keV cm²]. This quantity relates to the thermodynamic entropy as

where , the density of states in statistical theory, takes on the value of K as defined above.

See also

• Fossil group
• List of galaxy clusters
• Large-scale structure of the cosmos
• Timeline of galaxies, clusters of galaxies, and large-scale structure
• Intracluster medium

Gravitation is a natural phenomenon by which all objects with mass attract each other. In everyday life, gravitation is most familiar as the agency that endows objects with weight.
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large-scale structure refers to the characterization of observable distributions of matter and light on the largest scales (typically on the order of billions of light-years).
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Cold Dark Matter is a remixed, reedited CD version of the Psychic TV album Themes, originally included with WEA/Some Bizzare 12" vinyl release of Force The Hand Of Chance.

Credits

Produced, Composed, Mixed and Edited by Genesis P-Orridge.
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Super clusters are large groupings of smaller galaxy groups and clusters, and are among the largest structures of the cosmos. The existence of superclusters indicates that the galaxies in our Universe are not uniformly distributed; most of them are drawn together in groups and
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A galaxy (from the Greek root γαλαξίας, meaning "milky", a reference to our own Milky Way) is a massive, gravitationally bound system consisting of stars, an interstellar medium of gas and dust, and dark matter.
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Distances shorter than 10²² m

• 2.4×10²² m — 2.5 million light years — Distance to the Andromeda Galaxy
• 3.08568×10²² m — 3.2616 million light years — 1 megaparsec
• 4.0×10²² m — 4.

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The solar mass is a standard way to express mass in astronomy, used to describe the masses of other stars and galaxies. It is equal to the mass of the Sun, about two nonillion kilograms or about 332,950 times the mass of the Earth.
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Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias) sometimes referred to simply as "the Galaxy"), is a barred spiral galaxy that lies with the Local Group of galaxies
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Local Group is the group of galaxies that includes our galaxy, the Milky Way. The group comprises over 30 galaxies, with its gravitational center located somewhere between the Milky Way and the Andromeda Galaxy.
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In astronomy, the intracluster medium (or ICM) is the superheated gas present at the center of a galaxy cluster.

The ICM is composed primarily of baryons, i.e. ordinary matter; most of a cluster's baryons (about 80-95%) reside in the ICM, rather than in the luminous matter,
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Bremsstrahlung (pronounced ] (help) , from German bremsen "to brake" and
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atomic spectral lines are of two types:

• An emission line is formed when an electron makes a transition from a particular discrete energy level of an atom, to a lower energy state, emitting a photon of a particular energy and wavelength.

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Hydrostatic equilibrium occurs when compression due to gravity is balanced by a pressure gradient which creates a pressure gradient force in the opposite direction. The balance of these two forces is known as the hydrostatic balance.
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In astrophysics and cosmology, dark matter is hypothetical matter of unknown composition that does not emit or reflect enough electromagnetic radiation to be observed directly, but whose presence can be inferred from gravitational effects on visible matter.
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In astrophysics and cosmology, dark matter is hypothetical matter of unknown composition that does not emit or reflect enough electromagnetic radiation to be observed directly, but whose presence can be inferred from gravitational effects on visible matter.
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Distances shorter than 10²³ m

• 1.4×10²³ m — 15 million light years — Distance to Centaurus A galaxy
• 2.5×10²³ m — 27 million light years — Distance to the Pinwheel Galaxy
• 2.

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Virgo Cluster

Observation data
(Epoch J2000)
Constellation(s): Virgo & Coma Berenices
Right ascension: 12^h 27^m[1]
Declination:
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Hercules Cluster

Observation data
(Epoch J2000)
Constellation(s): Hercules
Right ascension: 16^h 05^m 15.
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Coma Cluster
A Sloan Digital Sky Survey/Spitzer Space Telescope image
of the Coma Cluster in ultraviolet and visible light
Courtesy: NASA/JPL-Caltech/GSFC/SDSS
Observation data
(Epoch J2000)
Constellation(s): Coma Berenices
Right ascension:
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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 galaxies, observable by its effect on the motion of galaxies and their
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Norma Cluster

Observation data
(Epoch J2000)
Constellation(s):
Right ascension: 16^h 15^m 32.
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Hubble's law is a statement in physical cosmology which states that the redshift in light coming from distant galaxies is proportional to their distance. The law was first formulated by Edwin Hubble and Milton Humason in 1929^[1] after nearly a decade of observations.
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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.
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Star clusters are groups of stars which are gravitationally bound. Two distinct types of star cluster can be distinguished: globular clusters are tight groups of hundreds of thousands of very old stars, while open clusters generally contain less than a few hundred members, and are
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open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud, and are still loosely gravitationally bound to each other. In contrast, globular clusters are very tightly bound by gravity.
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open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud, and are still loosely gravitationally bound to each other. In contrast, globular clusters are very tightly bound by gravity.
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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.
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Super clusters are large groupings of smaller galaxy groups and clusters, and are among the largest structures of the cosmos. The existence of superclusters indicates that the galaxies in our Universe are not uniformly distributed; most of them are drawn together in groups and
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Super clusters are large groupings of smaller galaxy groups and clusters, and are among the largest structures of the cosmos. The existence of superclusters indicates that the galaxies in our Universe are not uniformly distributed; most of them are drawn together in groups and
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Filament has a number of meanings in astronomy. For other uses, see filament.

• A galaxy filament
• A structure in a solar corona (solar filament)
• A Birkeland current

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