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.
Formation scenarios for BCGs include:
The study of accretion populations in BCGshas cast doubt over this theory and astronomers have seen no evidence of cooling flows in radiative cooling clusters. 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.The merging model is now generally accepted as the most likely one, but recent observations are at odds with some of its predictions. For example, it has been found that the stellar mass of BCGs was assembled much earlier than the merging model predicts.
BCGs are divided into various classes of galaxies: giant ellipticals (gE), D galaxies and cD galaxies.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.
The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time, and the processes that have generated the variety of structures observed in nearby galaxies. Galaxy formation is hypothesized to occur from structure formation theories, as a result of tiny quantum fluctuations in the aftermath of the Big Bang. The simplest model in general agreement with observed phenomena is the Lambda-CDM model—that is, that clustering and merging allows galaxies to accumulate mass, determining both their shape and structure.
A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. The word is derived from the Greek galaxias (γαλαξίας), literally "milky", a reference to the Milky Way. Galaxies range in size from dwarfs with just a few hundred million stars to giants with one hundred trillion stars, each orbiting its galaxy's center of mass.
A globular cluster is a spherical collection of stars. Globular clusters are very tightly bound by gravity, giving them their spherical shapes and high concentrations of stars toward their centers. Their name is derived from Latin globulus—a small sphere. Globular clusters are occasionally known simply as globulars.
An elliptical galaxy is a type of galaxy with an approximately ellipsoidal shape and a smooth, nearly featureless image. They are one of the four main classes of galaxy described by Edwin Hubble in his Hubble sequence and 1936 work The Realm of the Nebulae, along with spiral and lenticular galaxies. Elliptical (E) galaxies are, together with lenticular galaxies (S0) with their large-scale disks, and ES galaxies with their intermediate scale disks, a subset of the "early-type" galaxy population.
A lenticular galaxy is a type of galaxy intermediate between an elliptical and a spiral galaxy in galaxy morphological classification schemes. It contains a large-scale disc but does not have large-scale spiral arms. Lenticular galaxies are disc galaxies that have used up or lost most of their interstellar matter and therefore have very little ongoing star formation. They may, however, retain significant dust in their disks. As a result, they consist mainly of aging stars. Despite the morphological differences, lenticular and elliptical galaxies share common properties like spectral features and scaling relations. Both can be considered early-type galaxies that are passively evolving, at least in the local part of the Universe. Connecting the E galaxies with the S0 galaxies are the ES galaxies with intermediate-scale discs.
Dwarf elliptical galaxies, or dEs, are elliptical galaxies that are smaller than ordinary elliptical galaxies. They are quite common in galaxy groups and clusters, and are usually companions to other galaxies.
In astronomy, a galactic bulge is a tightly packed group of stars within a larger star formation. The term almost exclusively refers to the central group of stars found in most spiral galaxies. Bulges were historically thought to be elliptical galaxies that happened to have a disk of stars around them, but high-resolution images using the Hubble Space Telescope have revealed that many bulges lie at the heart of a spiral galaxy. It is now thought that there are at least two types of bulges: bulges that are like ellipticals and bulges that are like spiral galaxies.
A starburst galaxy is a galaxy undergoing an exceptionally high rate of star formation, as compared to the long-term average rate of star formation in the galaxy or the star formation rate observed in most other galaxies. For example, the star formation rate of the Milky Way galaxy is approximately 3 M☉/yr; however, starburst galaxies can experience star formation rates that are more than a factor of 33 times greater. In a starburst galaxy, the rate of star formation is so large that the galaxy will consume all of its gas reservoir, from which the stars are forming, on a timescale much shorter than the age of the galaxy. As such, the starburst nature of a galaxy is a phase, and one that typically occupies a brief period of a galaxy's evolution. The majority of starburst galaxies are in the midst of a merger or close encounter with another galaxy. Starburst galaxies include M82, NGC 4038/NGC 4039, and IC 10.
A mass deficit is the amount of mass that has been removed from the center of a galaxy, presumably by the action of a binary supermassive black hole.
A satellite galaxy is a smaller companion galaxy that travels on bound orbits within the gravitational potential of a more massive and luminous host galaxy. Satellite galaxies and their constituents are bound to their host galaxy, in the same way that planets within our own solar system are gravitationally bound to the Sun. While most satellite galaxies are dwarf galaxies, satellite galaxies of large galaxy clusters can be much more massive. The Milky Way is orbited by about fifty satellite galaxies, the largest of which is the Large Magellanic Cloud.
Interacting galaxies are galaxies whose gravitational fields result in a disturbance of one another. An example of a minor interaction is a satellite galaxy disturbing the primary galaxy's spiral arms. An example of a major interaction is a galactic collision, which may lead to a galaxy merger.
Galaxy mergers can occur when two galaxies collide. They are the most violent type of galaxy interaction. The gravitational interactions between galaxies and the friction between the gas and dust have major effects on the galaxies involved. The exact effects of such mergers depend on a wide variety of parameters such as collision angles, speeds, and relative size/composition, and are currently an extremely active area of research. Galaxy mergers are important because the merger rate is a fundamental measurement of galaxy evolution. The merger rate also provides astronomers with clues about how galaxies bulked up over time.
The M–sigmarelation is an empirical correlation between the stellar velocity dispersion σ of a galaxy bulge and the mass M of the supermassive black hole at its center.
The Sérsic profile is a mathematical function that describes how the intensity of a galaxy varies with distance from its center. It is a generalization of de Vaucouleurs' law. José Luis Sérsic first published his law in 1963.
Galactic clusters are gravitationally bound large-scale structures of multiple galaxies. The evolution of these aggregates is determined by time and manner of formation and the process of how their structures and constituents have been changing with time. Gamow (1952) and Weizscker (1951) showed that the observed rotations of galaxies are important for cosmology. They postulated that the rotation of galaxies might be a clue of physical conditions under which these systems formed. Thus, understanding the distribution of spatial orientations of the spin vectors of galaxies is critical to understanding the origin of the angular momenta of galaxies.
The type-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.
A2261-BCG 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 one of the largest galactic cores ever observed, spanning more than 10,000 light-years.
NGC 2623/Arp 243 is an interacting galaxy located in the constellation Cancer. NGC 2623 is the result of two spiral galaxies that have merged. Scientists believe that this situation is similar to what will occur to the Milky Way, which contains our solar system, and the neighboring galaxy, the Andromeda Galaxy in four billion years. Studying this galaxy and its properties have provided scientists with a better idea of the coming collision of the Milky Way and the Andromeda. Due to NGC 2623 being in the late stage of merging, the compression of the gas within the galaxy has led to a large amount of star formation, and to its unique structure of a bright core with two extending tidal tails.
NGC 3311 is a supergiant elliptical galaxy located about 190 million light-years away in the constellation Hydra. The galaxy was discovered by astronomer John Herschel on March 30, 1835. NGC 3311 is the brightest member of the Hydra Cluster and forms a pair with NGC 3309 which along with NGC 3311, dominate the central region of the Hydra Cluster.
NGC 1395 is an elliptical galaxy located in the constellation Eridanus. It is located at a distance of circa 75 million light years from Earth, which, given its apparent dimensions, means that NGC 1395 is about 130,000 light years across. It was discovered by William Herschel on November 17, 1784. It is a member of the Eridanus Cluster.