A lenticular galaxy is a type of galaxy intermediate between an elliptical and a spiral galaxy in galaxy morphological classification schemes. They contain large-scale discs but they do 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 little ongoing star formation, they may, retain significant dust in their disks. As a result, they consist of aging stars. Despite the morphological differences 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. Lenticular galaxies are unique in that they have a visible disk component as well as a prominent bulge component, they have much higher bulge-to-disk ratios than typical spirals and do not have the canonical spiral arm structure of late-type galaxies, yet may exhibit a central bar.
This bulge dominance can be seen in the axis ratio distribution of a lenticular galaxy sample. The distribution for lenticular galaxies rises in the range 0.25 to 0.85 whereas the distribution for spirals is flat in that same range. Larger axial ratios can be explained by observing face-on disk galaxies or by having a sample of spheroidal galaxies. Imagine looking at two disk galaxies edge-on, one with a bulge and one without a bulge; the galaxy with a prominent bulge will have a larger edge-on axial ratio compared to the galaxy without a bulge based on the definition of axial ratio. Thus a sample of disk galaxies with prominent spheroidal components will have more galaxies at larger axial ratios; the fact that the lenticular galaxy distribution rises with increasing observed axial ratio implies that lenticulars are dominated by a central bulge component. Lenticular galaxies are considered to be a poorly understood transition state between spiral and elliptical galaxies, which results in their intermediate placement on the Hubble sequence.
This results from lenticulars having bulge components. The disk component is featureless, which precludes a classification system similar to spiral galaxies; as the bulge component is spherical, elliptical galaxy classifications are unsuitable. Lenticular galaxies are thus divided into subclasses based upon either the amount of dust present or the prominence of a central bar; the classes of lenticular galaxies with no bar are S01, S02, S03 where the subscripted numbers indicate the amount of dust absorption in the disk component. The surface brightness profiles of lenticular galaxies are well described by the sum of a Sérsic model for the spheroidal component plus an exponentially declining model for the disk, a third component for the bar. Sometimes there is an observed truncation in the surface brightness profiles of lenticular galaxies at ~ 4 disk scalelengths; these features are consistent with the general structure of spiral galaxies. However, the bulge component of lenticulars is more related to elliptical galaxies in terms of morphological classification.
This spheroidal region, which dominates the inner structure of lenticular galaxies, has a steeper surface brightness profile than the disk component. Lenticular galaxy samples are distinguishable from the diskless elliptical galaxy population through analysis of their surface brightness profiles. Like spiral galaxies, lenticular galaxies can possess a central bar structure. While the classification system for normal lenticulars depends on dust content, barred lenticular galaxies are classified by the prominence of the central bar. SB01 galaxies have the least defined bar structure and are only classified as having enhanced surface brightness along opposite sides of the central bulge; the prominence of the bar increases with index number, thus SB03 galaxies have well defined bars that can extend through the transition region between the bulge and disk. The properties of bars in lenticular galaxies have not been researched in great detail. Understanding these properties, as well as understanding the formation mechanism for bars, would help clarify the formation or evolution history of lenticular galaxies.
In many respects the composition of lenticular galaxies is like that of ellipticals. For example, they both consist of predominately older, hence redder, stars. All of their stars are thought to be older than about a billion years, in agreement with their offset from the Tully–Fisher relation. In addition to these general stellar attributes, globular clusters are found more in lenticular galaxies than in spiral galaxies of similar mass and luminosity, they have little to no molecular gas and no significant hydrogen α or 21-cm emission. Unlike ellipticals, they may still possess significant dust. Lenticular galaxies share kinematic properties with elliptical galaxies; this is due to the significant disk nature of lenticulars. The bulge component is similar to elliptical galaxies in that it is pressure supported by a central velocity dispersion; this situation is analogous to a balloon, where the motions of the air particles are dominated by random motions. However, the kinematics of lenticular galaxies are dominated
H II region
An H II region or HII region is a region of interstellar atomic hydrogen, ionized. It is a cloud of ionized gas in which star formation has taken place, with a size ranging from one to hundreds of light years, density from a few to about a million particles per cubic cm; the Orion Nebula, now known to be an H II region, was observed in 1610 by Nicolas-Claude Fabri de Peiresc by telescope, the first such object discovered. They may be of any shape; the short-lived blue stars created in these regions emit copious amounts of ultraviolet light that ionize the surrounding gas. H II regions—sometimes several hundred light-years across—are associated with giant molecular clouds, they appear clumpy and filamentary, sometimes showing intricate shapes such as the Horsehead Nebula. H II regions may give birth to thousands of stars over a period of several million years. In the end, supernova explosions and strong stellar winds from the most massive stars in the resulting star cluster will disperse the gases of the H II region, leaving behind a cluster of stars which have formed, such as the Pleiades.
H II regions can be observed at considerable distances in the universe, the study of extragalactic H II regions is important in determining the distance and chemical composition of galaxies. Spiral and irregular galaxies contain many H II regions, while elliptical galaxies are devoid of them. In spiral galaxies, including our Milky Way, H II regions are concentrated in the spiral arms, while in irregular galaxies they are distributed chaotically; some galaxies contain huge H II regions. Examples include the 30 Doradus region in the Large Magellanic Cloud and NGC 604 in the Triangulum Galaxy; the term H II is pronounced "H two" by astronomers. "H" is the chemical symbol for hydrogen, "II" is the Roman numeral for 2. It is customary in astronomy to use the Roman numeral I for neutral atoms, II for singly-ionised—H II is H+ in other sciences—III for doubly-ionised, e.g. O III is O++, etc. H II, or H+, consists of free protons. An H I region being neutral atomic hydrogen, a molecular cloud being molecular hydrogen, H2.
In spoken discussion with non-astronomers there is sometimes confusion between the identical spoken forms of "H II" and "H2". A few of the brightest H II regions are visible to the naked eye. However, none seem to have been noticed before the advent of the telescope in the early 17th century. Galileo did not notice the Orion Nebula when he first observed the star cluster within it; the French observer Nicolas-Claude Fabri de Peiresc is credited with the discovery of the Orion Nebula in 1610. Since that early observation large numbers of H II regions have been discovered in the Milky Way and other galaxies. William Herschel observed the Orion Nebula in 1774, described it as "an unformed fiery mist, the chaotic material of future suns". In early days astronomers distinguished between "diffuse nebulae", which retained their fuzzy appearance under magnification through a large telescope, nebulae that could be resolved into stars, now known to be galaxies external to our own. Confirmation of Herschel's hypothesis of star formation had to wait another hundred years, when William Huggins together with his wife Mary Huggins turned his spectroscope on various nebulae.
Some, such as the Andromeda Nebula, had spectra quite similar to those of stars, but turned out to be galaxies consisting of hundreds of millions of individual stars. Others looked different. Rather than a strong continuum with absorption lines superimposed, the Orion Nebula and other similar objects showed only a small number of emission lines. In planetary nebulae, the brightest of these spectral lines was at a wavelength of 500.7 nanometres, which did not correspond with a line of any known chemical element. At first it was hypothesized that the line might be due to an unknown element, named nebulium—a similar idea had led to the discovery of helium through analysis of the Sun's spectrum in 1868. However, while helium was isolated on earth soon after its discovery in the spectrum of the sun, nebulium was not. In the early 20th century, Henry Norris Russell proposed that rather than being a new element, the line at 500.7 nm was due to a familiar element in unfamiliar conditions. Interstellar matter, considered dense in an astronomical context, is at high vacuum by laboratory standards.
Physicists showed in the 1920s that in gas at low density, electrons can populate excited metastable energy levels in atoms and ions, which at higher densities are de-excited by collisions. Electron transitions from these levels in doubly ionized oxygen give rise to the 500.7 nm line. These spectral lines, which can only be seen in low density gases, are called forbidden lines. Spectroscopic observations thus showed that planetary nebulae consisted of rarefied ionised oxygen gas. During the 20th century, observations showed that H II regions contained hot, bright stars; these stars are many times more massive than the Sun, are the shortest-lived stars, with total lifetimes of only a few million years. Therefore, it was surmised. Over a period of several million years, a cluster of stars will form in an H II region, before radiation pressure from the hot young stars causes the nebula to disperse; the Pleiades are an example of a cluster which has'boiled away' the H II region from which it was formed.
Only a trace of reflection nebulosity remain
Interacting galaxies are galaxies whose gravitational fields result in a disturbance of one another. An example of a minor interaction is a satellite galaxy's disturbing the primary galaxy's spiral arms. An example of a major interaction is a galactic collision. A giant galaxy interacting with its satellites is common. A satellite's gravity could attract one of the primary's spiral arms, or the secondary satellite's path could coincide with the position of the primary satellite's and so would dive into the primary galaxy; that can trigger a small amount of star formation. Such orphaned clusters of stars were sometimes referred to as "blue blobs" before they were recognized as stars. Colliding galaxies are common during galaxy evolution; the tenuous distribution of matter in galaxies means these are not collisions in the traditional sense of the word, but rather gravitational interactions. Colliding may lead to merging if two galaxies collide and do not have enough momentum to continue traveling after the collision.
In that case, they fall back into each other and merge into one galaxy after many passes through each other. If one of the colliding galaxies is much larger than the other, it will remain intact after the merger; the larger galaxy will look much the same, while the smaller galaxy will be stripped apart and become part of the larger galaxy. When galaxies pass through each other, unlike during mergers, they retain their material and shape after the pass. Galactic collisions are now simulated on computers, which use realistic physics principles, including the simulation of gravitational forces, gas dissipation phenomena, star formation, feedback. Dynamical friction slows the relative motion galaxy pairs, which may merge at some point, according to the initial relative energy of the orbits. A library of simulated galaxy collisions can be found at the Paris Observatory website: GALMER Galactic cannibalism refers to the process in which a large galaxy, through tidal gravitational interactions with a companion, merges with that companion.
The most common result of the gravitational merger between two or more galaxies is an irregular galaxy, but elliptical galaxies may result. It has been suggested that galactic cannibalism is occurring between the Milky Way and the Large and Small Magellanic Clouds. Streams of gravitationally-attracted hydrogen arcing from these dwarf galaxies to the Milky Way is taken as evidence for the theory. Galaxy harassment is a type of interaction between a low-luminosity galaxy and a brighter one that takes place within rich galaxy clusters, such as Virgo and Coma, where galaxies are moving at high relative speeds and suffering frequent encounters with other systems of the cluster by the high galactic density of the latter. According to computer simulations, the interactions convert the affected galaxy disks into disturbed barred spiral galaxies and produces starbursts followed by, if more encounters occur, loss of angular momentum and heating of their gas; the result would be the conversion of low-luminosity spiral galaxies into dwarf spheroidals and dwarf ellipticals.
Evidence for the hypothesis had been claimed by studying early-type dwarf galaxies in the Virgo Cluster and finding structures, such as disks and spiral arms, which suggest they are former disk systems transformed by the above-mentioned interactions. However, the existence of similar structures in isolated early-type dwarf galaxies, such as LEDA 2108986, has undermined this hypothesis Astronomers have estimated the Milky Way galaxy, will collide with the Andromeda galaxy in about 4.5 billion years. It is thought that the two spiral galaxies will merge to become an elliptical galaxy or a large disk galaxy. NGC 7318 Galaxy Collisions Galactic cannibalism Galactic Collision Simulation GALMER: Galaxy Merger Simulations
A globular cluster is a spherical collection of stars that orbit a galactic core, as a satellite. Globular clusters are tightly bound by gravity, which gives them their spherical shapes, high stellar densities toward their centers; the name of this category of star cluster is derived from globulus -- a small sphere. A globular cluster is sometimes known, more as a globular. Globular clusters are found in the halo of a galaxy and contain more stars, are much older, than the less dense, open clusters which are found in the disk of a galaxy. Globular clusters are common. Larger galaxies can have more: The Andromeda Galaxy, for instance, may have as many as 500; some giant elliptical galaxies, such as M87, have as many as 13,000 globular clusters. Every galaxy of sufficient mass in the Local Group has an associated group of globular clusters, every large galaxy surveyed, has been found to possess a system of globular clusters; the Sagittarius Dwarf galaxy, the disputed Canis Major Dwarf galaxy appear to be in the process of donating their associated globular clusters to the Milky Way.
This demonstrates. Although it appears that globular clusters contain some of the first stars to be produced in the galaxy, their origins and their role in galactic evolution are still unclear, it does appear clear that globular clusters are different from dwarf elliptical galaxies and were formed as part of the star formation of the parent galaxy, rather than as a separate galaxy. The first known globular cluster, now called M22, was discovered in 1665 by Abraham Ihle, a German amateur astronomer. However, given the small aperture of early telescopes, individual stars within a globular cluster were not resolved until Charles Messier observed M4 in 1764; the first eight globular clusters discovered are shown in the table. Subsequently, Abbé Lacaille would list NGC 104, NGC 4833, M55, M69, NGC 6397 in his 1751–52 catalogue; the M before a number refers to Charles Messier's catalogue, while NGC is from the New General Catalogue by John Dreyer. When William Herschel began his comprehensive survey of the sky using large telescopes in 1782 there were 34 known globular clusters.
Herschel discovered another 36 himself and was the first to resolve all of them into stars. He coined the term "globular cluster" in his Catalogue of a Second Thousand New Nebulae and Clusters of Stars published in 1789; the number of globular clusters discovered continued to increase, reaching 83 in 1915, 93 in 1930 and 97 by 1947. A total of 152 globular clusters have now been discovered in the Milky Way galaxy, out of an estimated total of 180 ± 20; these additional, undiscovered globular clusters are believed to be hidden behind the gas and dust of the Milky Way. Beginning in 1914, Harlow Shapley began a series of studies of globular clusters, published in about 40 scientific papers, he examined the RR Lyrae variables in the clusters and used their period–luminosity relationship for distance estimates. It was found that RR Lyrae variables are fainter than Cepheid variables, which caused Shapley to overestimate the distances of the clusters. Of the globular clusters within the Milky Way, the majority are found in a halo around the galactic core, the large majority are located in the celestial sky centered on the core.
In 1918, this asymmetrical distribution was used by Shapley to make a determination of the overall dimensions of the galaxy. By assuming a spherical distribution of globular clusters around the galaxy's center, he used the positions of the clusters to estimate the position of the Sun relative to the galactic center. While his distance estimate was in significant error, it did demonstrate that the dimensions of the galaxy were much greater than had been thought, his error was due to interstellar dust in the Milky Way, which absorbs and diminishes the amount of light from distant objects, such as globular clusters, that reaches the Earth, thus making them appear to be more distant than they are. Shapley's measurements indicated that the Sun is far from the center of the galaxy contrary to what had been inferred from the nearly distribution of ordinary stars. In reality, most ordinary stars lie within the galaxy's disk and those stars that lie in the direction of the galactic centre and beyond are thus obscured by gas and dust, whereas globular clusters lie outside the disk and can be seen at much further distances.
Shapley was subsequently assisted in his studies of clusters by Henrietta Swope and Helen Battles Sawyer. In 1927–29, Shapley and Sawyer categorized clusters according to the degree of concentration each system has toward its core; the most concentrated clusters were identified as Class I, with successively diminishing concentrations ranging to Class XII. This became known as the Shapley–Sawyer Concentration Class In 2015, a new type of globular cluster was proposed on the basis of observational data, the dark globular clusters; the formation of globular clusters remains a poorly understood phenomenon and it remains uncertain whether the stars in a globular cluster form in a single generation or are spawned across multiple generations over a period of several hundred million years. In many globular clusters, most of the stars are at approxima
A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas and dark matter. The word galaxy is derived from the Greek galaxias "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. Galaxies are categorized according to their visual morphology as spiral, or irregular. Many galaxies are thought to have supermassive black holes at their centers; the Milky Way's central black hole, known as Sagittarius A*, has a mass four million times greater than the Sun. As of March 2016, GN-z11 is the oldest and most distant observed galaxy with a comoving distance of 32 billion light-years from Earth, observed as it existed just 400 million years after the Big Bang. Research released in 2016 revised the number of galaxies in the observable universe from a previous estimate of 200 billion to a suggested 2 trillion or more, containing more stars than all the grains of sand on planet Earth.
Most of the galaxies are 1,000 to 100,000 parsecs in diameter and separated by distances on the order of millions of parsecs. For comparison, the Milky Way has a diameter of at least 30,000 parsecs and is separated from the Andromeda Galaxy, its nearest large neighbor, by 780,000 parsecs; the space between galaxies is filled with a tenuous gas having an average density of less than one atom per cubic meter. The majority of galaxies are gravitationally organized into groups and superclusters; the Milky Way is part of the Local Group, dominated by it and the Andromeda Galaxy and is part of the Virgo Supercluster. At the largest scale, these associations are arranged into sheets and filaments surrounded by immense voids; the largest structure of galaxies yet recognised is a cluster of superclusters, named Laniakea, which contains the Virgo supercluster. The origin of the word galaxy derives from the Greek term for the Milky Way, galaxias, or kyklos galaktikos due to its appearance as a "milky" band of light in the sky.
In Greek mythology, Zeus places his son born by a mortal woman, the infant Heracles, on Hera's breast while she is asleep so that the baby will drink her divine milk and will thus become immortal. Hera wakes up while breastfeeding and realizes she is nursing an unknown baby: she pushes the baby away, some of her milk spills, it produces the faint band of light known as the Milky Way. In the astronomical literature, the capitalized word "Galaxy" is used to refer to our galaxy, the Milky Way, to distinguish it from the other galaxies in our universe; the English term Milky Way can be traced back to a story by Chaucer c. 1380: "See yonder, lo, the Galaxyë Which men clepeth the Milky Wey, For hit is whyt." Galaxies were discovered telescopically and were known as spiral nebulae. Most 18th to 19th Century astronomers considered them as either unresolved star clusters or anagalactic nebulae, were just thought as a part of the Milky Way, but their true composition and natures remained a mystery. Observations using larger telescopes of a few nearby bright galaxies, like the Andromeda Galaxy, began resolving them into huge conglomerations of stars, but based on the apparent faintness and sheer population of stars, the true distances of these objects placed them well beyond the Milky Way.
For this reason they were popularly called island universes, but this term fell into disuse, as the word universe implied the entirety of existence. Instead, they became known as galaxies. Tens of thousands of galaxies have been catalogued, but only a few have well-established names, such as the Andromeda Galaxy, the Magellanic Clouds, the Whirlpool Galaxy, the Sombrero Galaxy. Astronomers work with numbers from certain catalogues, such as the Messier catalogue, the NGC, the IC, the CGCG, the MCG and UGC. All of the well-known galaxies appear in one or more of these catalogues but each time under a different number. For example, Messier 109 is a spiral galaxy having the number 109 in the catalogue of Messier, having the designations NGC 3992, UGC 6937, CGCG 269-023, MCG +09-20-044, PGC 37617; the realization that we live in a galaxy, one among many galaxies, parallels major discoveries that were made about the Milky Way and other nebulae. The Greek philosopher Democritus proposed that the bright band on the night sky known as the Milky Way might consist of distant stars.
Aristotle, believed the Milky Way to be caused by "the ignition of the fiery exhalation of some stars that were large and close together" and that the "ignition takes place in the upper part of the atmosphere, in the region of the World, continuous with the heavenly motions." The Neoplatonist philosopher Olympiodorus the Younger was critical of this view, arguing that if the Milky Way is sublunary it should appear different at different times and places on Earth, that it should have parallax, which it does not. In his view, the Milky Way is celestial. According to Mohani Mohamed, the Arabian astronomer Alhazen made the first attempt at observing and measuring the Milky Way's parallax, he thus "determined that because the Milky Way had no parallax, it must be remote from the Earth, not belonging to the atmosphere." The Persian astronomer al-Bīrūnī
Spiral galaxies form a class of galaxy described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, form part of the Hubble sequence. Most spiral galaxies consist of a flat, rotating disk containing stars and dust, a central concentration of stars known as the bulge; these are surrounded by a much fainter halo of stars, many of which reside in globular clusters. Spiral galaxies are named by their spiral structures that extend from the center into the galactic disc; the spiral arms are sites of ongoing star formation and are brighter than the surrounding disc because of the young, hot OB stars that inhabit them. Two-thirds of all spirals are observed to have an additional component in the form of a bar-like structure, extending from the central bulge, at the ends of which the spiral arms begin; the proportion of barred spirals relative to their barless cousins has changed over the history of the Universe, with only about 10% containing bars about 8 billion years ago, to a quarter 2.5 billion years ago, until present, where over two-thirds of the galaxies in the visible universe have bars.
Our own Milky Way is a barred spiral, although the bar itself is difficult to observe from the Earth's current position within the galactic disc. The most convincing evidence for the stars forming a bar in the galactic center comes from several recent surveys, including the Spitzer Space Telescope. Together with irregular galaxies, spiral galaxies make up 60% of galaxies in today's universe, they are found in low-density regions and are rare in the centers of galaxy clusters. Spiral galaxies may consist of several distinct components: A flat, rotating disc of stars and interstellar matter of which spiral arms are prominent components A central stellar bulge of older stars, which resembles an elliptical galaxy A bar-shaped distribution of stars A near-spherical halo of stars, including many in globular clusters A supermassive black hole at the center of the central bulge A near-spherical dark matter haloThe relative importance, in terms of mass and size, of the different components varies from galaxy to galaxy.
Spiral arms are regions of stars that barred spiral galaxies. These long, thin regions resemble a spiral and thus give spiral galaxies their name. Different classifications of spiral galaxies have distinct arm-structures. Sc and SBc galaxies, for instance, have "loose" arms, whereas Sa and SBa galaxies have wrapped arms. Either way, spiral arms contain many blue stars, which make the arms so bright. A bulge is a large packed group of stars; the term refers to the central group of stars found in most spiral galaxies defined as the excess of stellar light above the inward extrapolation of the outer disk light. Using the Hubble classification, the bulge of Sa galaxies is composed of Population II stars, which are old, red stars with low metal content. Further, the bulge of Sa and SBa galaxies tends to be large. In contrast, the bulges of Sc and SBc galaxies are much smaller and are composed of young, blue Population I stars; some bulges have similar properties to those of elliptical galaxies. Many bulges are thought to host a supermassive black hole at their centers.
As of April 10th, 2019 the existence of these supermassive black holes was confirmed when scientists released the first image of a black hole in the center of the Messier 87 galaxy. In our own galaxy, for instance, the object called Sagittarius A* is believed to be a supermassive black hole. Bar-shaped elongations of stars are observed in two-thirds of all spiral galaxies, their presence may be either weak. In edge-on spiral galaxies, the presence of the bar can sometimes be discerned by the out-of-plane X-shaped or -shaped structures which have a maximum visibility at half the length of the in-plane bar; the bulk of the stars in a spiral galaxy are located either close to a single plane in more or less conventional circular orbits around the center of the galaxy, or in a spheroidal galactic bulge around the galactic core. However, some stars inhabit a type of galactic halo; the orbital behaviour of these stars is disputed, but they may exhibit retrograde and/or inclined orbits, or not move in regular orbits at all.
Halo stars may be acquired from small galaxies which fall into and merge with the spiral galaxy—for example, the Sagittarius Dwarf Spheroidal Galaxy is in the process of merging with the Milky Way and observations show that some stars in the halo of the Milky Way have been acquired from it. Unlike the galactic disc, the halo seems to be free of dust, in further contrast, stars in the galactic halo are of Population II, much older and with much lower metallicity than their Population I cousins in the galactic disc; the galactic halo contains many globular clusters. The motion of halo stars does bring them through the disc on occasion, a number of small red dwarfs close to the Sun are thought to belong to the galactic halo, for example Kapteyn's Star and Groombridge 1830. Due to their irregular movement around the center of the galaxy, these stars display unusually high proper motion; the oldest spiral galaxy on file is BX442. At eleven billion years old, it is more than two billion years older than any previous discovery.
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 gravitational formation of structure with cold dark matter, the smallest structures collapse first and build the largest structures, clusters of galaxies. Clusters are formed recently between 10 billion years ago and now. Groups and clusters may contain ten to thousands of individual galaxies; the clusters themselves are associated with larger, non-gravitationally bound, groups called superclusters. Groups of galaxies are the smallest aggregates of galaxies, they contain no more than 50 galaxies in a diameter of 1 to 2 megaparsecs. Their mass is 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.
Groups are the most common structures of galaxies in the universe, comprising at least 50% of the galaxies in the local universe. Groups have a mass range between those of the large elliptical galaxies and clusters of galaxies. Our own Galaxy, the Milky Way, is contained in the Local Group of more than 40 galaxies. In July 2017 S. Paul, R. S. John et al. define clear distinguishing parameters for classifying ‘galaxy groups’ and ‘clusters’ on the basis of scaling laws that they followed. According to this paper, those large scale structures in the universe with mass less than 8 × 1013 solar mass is classified as Galaxy group. Clusters are larger than groups; 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 hot, between 107K and 108K, 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 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 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 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. Brownstein and Moffat use a theory of modified gravity to explain X-ray cluster masses without dark matter. Observations of the Bullet Cluster are the strongest evidence for the existence of dark matter.
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, confirmed by finding several galaxies at a similar redshift. Infrared searches are more useful for finding more distant clusters. X-ray: The hot plasma emits X-rays that can be detected by X-ray telescopes; the cluster gas can be studied using both X-ray 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 have been used as tracers of cluster location. At high redshift imaging around individual radio sources has been used to detect proto-clusters.
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. 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, thermal feedback triggered by that cooling.
The density 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 c