# Brightest cluster galaxy

A brightest cluster galaxy (BCG) is defined as the brightest galaxy in a cluster of galaxies. BCGs include the most massive galaxies in the universe. They are generally elliptical galaxies which lie close to the geometric and kinematical center of their host galaxy cluster, hence at the bottom of the cluster potential well. They are also generally coincident with the peak of the cluster X-ray emission.[1]

Formation scenarios for BCGs include:

• Cooling flow—Star formation from the central cooling flow in high density cooling centers of X-ray cluster halos.

The study of accretion populations in BCGs[2] has cast doubt over this theory and astronomers have seen no evidence of cooling flows in radiative cooling clusters.[3] The two remaining theories exhibit healthier prospects.

It is possible to differentiate the cannibalism model from the merging model by considering the formation period of the BCGs. In the cannibalism model, there are numerous small galaxies present in the evolved cluster, whereas in the merging model, a hierarchical cosmological model is expected due to the collapse of clusters. It has been shown that the orbit decay of cluster galaxies is not effective enough to account for the growth of BCGs.[6] The merging model is now generally accepted as the most likely one,[7] but recent observations are at odds with some of its predictions. For example, it has been found that the stellar mass of BCG's was assembled much earlier than the merging model predicts.[8]

The brightest galaxy in the image is named SDSS J1156+1911, taken by Hubble.[9]

BCGs are divided into various classes of galaxies: giant ellipticals (gE), D galaxies and cD galaxies.[10] cD and D galaxies both exhibit an extended diffuse envelope surrounding an elliptical-like nucleus akin to regular elliptical galaxies. The light profiles of BCGs are often described by a Sersic surface brightness law, a double Sersic profile or a de Vaucouleurs law. The different parametrizations of the light profile of BCG's, as well as the faintness of the diffuse envelope lead to discrepancies in the reported values of the sizes of these objects.

This image from NASA's Hubble Space Telescope shows the galaxy cluster Abell S0740 that is over 450 million light-years away in the direction of the constellation Centaurus. The giant elliptical galaxy ESO 325-G004 looms large at the cluster's center. This BCG is as massive as 100 billion of our suns.

## References

1. ^
2. ^ McNamara and O’Connell (1989), Star formation in cooling flows in clusters of galaxies
3. ^ Motl et al. (2004), Formation of Cool Cores in Galaxy Clusters via Hierarchical Mergers
4. ^ J. Ostriker and M. Hausman (1977), Cannibalism among the galaxies - Dynamically produced evolution of cluster luminosity functions
5. ^
6. ^
7. ^ J. Dubinski (1998), The Origin of the Brightest Cluster Galaxies
8. ^ Collins et al. (2009) Early assembly of the most massive galaxies
9. ^ "A green cosmic arc". www.spacetelescope.org. Retrieved 28 May 2018.
10. ^ Matthews, T. A., Morgan, W. W. and Schmidt, M. (1964),A Discussion of Galaxies Identified with Radio Sources

A2261-BCG

A2261-BCG (short for Abell 2261 Brightest Cluster Galaxy) is a huge elliptical galaxy in the cluster Abell 2261. One of the largest galaxies known, A2261-BCG is estimated to have a diameter of a million light-years, some 10 times larger than the Milky Way. It is the brightest and the most massive galaxy in the cluster, and has the largest galactic core ever observed, spanning more than 10,000 light-years.The cD elliptical galaxy, located at least 3 billion light-years from Earth, is also well known as a radio source. Its core is highly populated by a dense number of old stars, but is mysteriously diffuse, giving it a large core.

On September 10, 2012, using Hubble Space Telescope's Wide Field Camera 3, scientists found out that there was no supermassive black hole present in the center. This may be the likely cause of its diffuse and large core, but it contradicts modern galactic evolutionary theories. A huge cD galaxy like A2261-BCG would be expected to have a supermassive black hole concentrated at its center.

Abell 1201 BCG

Abell 1201 BGC is a massive elliptical galaxy

residing as the brightest cluster galaxy of the Abell 1201 galaxy cluster.

At a redshift of 0.169, this system is around 2.7 billion light years from Earth,

and offset about 11 kiloparsecs from the X-ray peak of the intracluster gas.

With an ellipticity of 0.32±0.02, the stellar distribution is far from spherical.

In solar units, the total stellar luminosity is

4×1011 L☉ in SDSS r-band,

and 1.6×1012 L☉ in 2MASS K-band.

Half the stars orbit within an effective radius of 15 kpc,

and their central velocity dispersion is about 285 km s−1 within 5 kpc

rising to 360 km s−1 at 20 kpc distance.The BCG also acts as a gravitational lens,

bending the light of a more distant background galaxy (at redshift 0.451)

into an apparent tangential arc about 6 kpc to one side.

This makes the galaxy an important case in investigations of the intrinsic properties of dark matter.

Detailed models of the lens mass distribution, starlight and stellar kinematics

indicates that the galaxy cluster's dark halo has a shallow inner density gradient and perhaps a soft dark matter core.

At face value, this is incompatible with the dark matter cusp predicted by collisionless Cold dark matter theories,

Years later, a faint smaller counterimage to the arc was discovered at a closer radius.

Explaining the position and brightness of this counterimage requires a dark central concentration of unseen mass.

Based on lens modelling, it could be a supermassive black hole equivalent to 13 billion suns:

(1.3±0.6)×1010 M☉.

At the time of measurement, this is one of the most massive black hole candidates

(without relying on assumptions about quasar luminosities and efficiencies).

This UMBH may be ten times larger than expected from

the usual scaling relations between black holes and host galaxies.However, alternative methods of modelling the stellar velocity dispersion maps

(accounting for an aggregate constraint on the lens mass)

reveals an ambiguity between the UMBH mass and the dark halo profile.

In solutions where the UMBH is more massive, the dark matter is more cuspy.

In solutions where the UMBH is smaller or absent, the dark matter is more cored.

The dark halo's ellipticity and the mass-to-light ratio of stars also enter the ambiguity.

Thus, if the standard computational models are robust,

then Abell 1201 BCG presents a dilemma and a challenge to either

the conventional ideas of black hole growth

or the simplest theories about dark matter, or both.

Abell 2029

Abell 2029 or A2029 is a large cluster of galaxies 315 megaparsecs (1.027 billion light-years) away in the constellation Virgo. A2029 is a Bautz–Morgan classification type I cluster due to its large central galaxy, IC 1101. Abell 2029 has a diameter of 5.8–8 million light-years. This type of galaxy is called a cD-type brightest cluster galaxy and may have grown to its large size by accreting nearby galaxies. Despite its relaxed state, it is the central member of a large supercluster which shows clear signs of interaction.

Abell 2199

Abell 2199 is a galaxy cluster in the Abell catalogue featuring a brightest cluster galaxy NGC 6166, a cD galaxy. Abell 2199 is the definition of a Bautz-Morgan type I cluster due to NGC 6166.

Abell 2261

Abell 2261 is one of 25 galaxy clusters being studied as part of the Cluster Lensing And Supernova survey with Hubble (CLASH) program, a major project to build a library of scientific data on lensing clusters.It also has the galaxy A2261-BCG (short for Abell 2261 Brightest Cluster Galaxy) which has the largest galaxy core ever observed.

Antlia Cluster

The Antlia Cluster (or Abell S0636) is a cluster of galaxies located in the Hydra-Centaurus Supercluster. The Antlia Cluster is the third nearest to the Local Group after the Virgo Cluster and Fornax Cluster. Antlia's distance from Earth is 40.5 Mpc (132.1 Mly) to 40.9 Mpc (133.4 Mly) and can be viewed from Earth in the constellation Antlia. The Antlia Cluster should not be confused with the Antlia Dwarf Galaxy.Antlia is classified as a rare Bautz-Morgan type III cluster, meaning it has no central dominant (cD) brightest cluster galaxy. However, the cluster is dominated by two massive elliptical galaxies, NGC 3268 and NGC 3258, and contains a total of about 234 galaxies. The cluster is very dense compared to other clusters such as Virgo and Fornax, thus containing early-type galaxies and a larger portion of dwarf ellipticals. The Cluster is split into two galaxy groups, The Northern subgroup gravitating around NGC 3268, and the Southern subgroup centered on NGC 3258.The cluster has an overall redshift of z = 0.0087, implying that the cluster is, like most objects in the Universe, receding from the Local Group. Using the now-obsolete scientific satellite ASCA, X-ray observations show that the cluster is almost isothermal, with a mean temperature of kT ~ 2.0 keV.

Coma Filament

Coma Filament is a galaxy filament. The filament contains the Coma Supercluster of galaxies and forms a part of the CfA2 Great Wall.

IC 1101

IC 1101 is a supergiant elliptical galaxy at the center of the Abell 2029 galaxy cluster, approximately 320 megaparsecs (1.04 billion light-years) from Earth.

LBG-2377

LBG-2377 is the most distant galaxy merger discovered, as of 2008, at a distance of 11.4 billion light years. This galaxy merger is so distant that the universe was in its infancy when its light was emitted. It is expected that this galaxy proto-cluster will merge to form a brightest cluster galaxy, and become the core of a larger galaxy cluster.

List of the most distant astronomical objects

This article documents the most distant astronomical objects so far discovered, and the time periods in which they were so classified.

Distances to remote objects, other than those in nearby galaxies, are nearly always inferred by measuring the cosmological redshift of their light. By their nature, very distant objects tend to be very faint, and these distance determinations are difficult and subject to errors. An important distinction is whether the distance is determined via spectroscopy or using a photometric redshift technique. The former is generally both more precise and also more reliable, in the sense that photometric redshifts are more prone to being wrong due to confusion with lower redshift sources that have unusual spectra. For that reason, a spectroscopic redshift is conventionally regarded as being necessary for an object's distance to be considered definitely known, whereas photometrically determined redshifts identify "candidate" very distant sources. Here, this distinction is indicated by a "p" subscript for photometric redshifts.

Lynx–Ursa Major Filament

Lynx–Ursa Major Filament (LUM Filament) is a galaxy filament.The filament is connected to and separate from the Lynx–Ursa Major Supercluster.

Mass deficit

A mass deficit is the amount of mass (in stars) that has been removed from the center of a galaxy, presumably by the action of a binary supermassive black hole.

The density of stars increases toward the center in most galaxies. In small galaxies, this increase continues into the very center. In large galaxies, there is usually a "core", a region near the center where the density is constant or slowly rising. The size of the core – the "core radius" – can be a few hundred parsecs in large elliptical galaxies. The greatest observed stellar cores reach 3.2 to 5.7 kiloparsecs in radius.

It is believed that cores are produced by binary supermassive black holes (SMBHs). Binary SMBHs form during the merger of two galaxies. If a star passes near the massive binary, it will be ejected, by a process called the gravitational slingshot. This ejection continues until most of the stars near the center of the galaxy have been removed. The result is a low-density core. Such cores are ubiquitous in giant elliptical galaxies.

The mass deficit is defined as the amount of mass that was removed in creating the core. Mathematically, the mass deficit is defined as

${\displaystyle M_{\mathrm {def} }=4\pi \int _{0}^{R_{c}}\left[\rho _{i}(r)-\rho (r)\right]r^{2}dr}$,

where ρi is the original density, ρ is the observed density, and Rc is the core radius. In practice, the core-Sersic model can be used to help quantify the deficits.

Observed mass deficits are typically in the range of one to a few times the mass of the central SMBH, and observed core radii are comparable to the influence radii of the central SMBH. These properties are consistent with what is predicted in theoretical models of core formation and lend support to the hypothesis that all bright galaxies once contained binary SMBHs at their centers.

It is not known whether most galaxies still contain massive binaries, or whether the two black holes have coalesced. Both possibilities are consistent with the presence of mass deficits.

NGC 6086

NGC 6086 is an elliptical galaxy in the constellation of Corona Borealis. It has an apparent magnitude of 12.7. A Type-cD galaxy, it is the brightest cluster galaxy in the cluster Abell 2162. In 2010, a supermassive black hole was discovered in NGC 6086.

NGC 80

NGC 80 is a lenticular galaxy located in the constellation Andromeda. It is interacting with NGC 47 and NGC 68, and is the brightest cluster galaxy of the NGC 80 group, a galaxy group named after it.

Perseus–Pegasus Filament

Perseus–Pegasus Filament is a galaxy filament containing the Perseus-Pisces Supercluster and stretching for roughly a billion light years (or over 300/h Mpc). Currently, it is considered to be one of the largest known structures in the universe. This filament is adjacent to the Pisces–Cetus Supercluster Complex.

Type-cD galaxy

The type-cD galaxy (also cD-type galaxy, cD galaxy) is a galaxy morphology classification, a subtype of type-D giant elliptical galaxy. Characterized by a large halo of stars, they can be found near the centres of some rich galaxy clusters. They are also known as supergiant ellipticals or central dominant galaxies.

Ursa Major Filament

Ursa Major Filament is a galaxy filament. The filament is connected to the CfA Homunculus, a portion of the filament forms a portion of the "leg" of the Homunculus.

Morphology
Structure
Active nuclei
Energetic galaxies
Low activity
Interaction
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