A galaxy group or group of galaxies ( GrG ) is an aggregation of galaxies comprising about 50 or fewer gravitationally bound members, each at least as luminous as the Milky Way (about 10 times the luminosity of the Sun ); collections of galaxies larger than groups that are first-order clustering are called galaxy clusters . The groups and clusters of galaxies can themselves be clustered, into superclusters of galaxies.
22-693: The M101 Group is a loose group of galaxies located in the constellation Ursa Major . The group is named after the brightest galaxy in the group, the Pinwheel Galaxy (M101). Most of the other members of the group are companions of the Pinwheel Galaxy. The group itself is one of many located within the Virgo Supercluster (i.e. the Local Supercluster ). The table below lists galaxies that have been consistently identified as group members in
44-520: A blue compact dwarf galaxy ( BCD galaxy ) is a small galaxy which contains large clusters of young, hot, massive stars . These stars, the brightest of which are blue, cause the galaxy itself to appear blue in colour. Most BCD galaxies are also classified as dwarf irregular galaxies or as dwarf lenticular galaxies . Because they are composed of star clusters, BCD galaxies lack a uniform shape. They consume gas intensely, which causes their stars to become very violent when forming. BCD galaxies cool in
66-523: A bound group. Compact galaxy groups are also not dynamically stable over Hubble time , thus showing that galaxies evolve by merger, over the timescale of the age of the universe. Fossil galaxy groups, fossil groups, or fossil clusters are believed to be the end-result of galaxy merging within a normal galaxy group, leaving behind the X-ray halo of the progenitor group. Galaxies within a group interact and merge. The physical process behind this galaxy-galaxy merger
88-539: A few hundred to one hundred thousand stars , making them the faintest galaxies in the Universe . UFDs resemble globular clusters (GCs) in appearance but have very different properties. Unlike GCs, UFDs contain a significant amount of dark matter and are more extended. UFDs were first discovered with the advent of digital sky surveys in 2005, in particular with the Sloan Digital Sky Survey (SDSS). UFDs are
110-518: A much later time than the ancient UFDs. These galaxies have not been observed in our Universe so far. Ultra-compact dwarf galaxies (UCD) are a class of very compact galaxies with very high stellar densities, discovered in the 2000s. They are thought to be on the order of 200 light years across, containing about 100 million stars. It is theorised that these are the cores of nucleated dwarf elliptical galaxies that have been stripped of gas and outlying stars by tidal interactions , travelling through
132-575: Is M60-UCD1 , about 54 million light years away, which contains approximately 200 million solar masses within a 160 light year radius; the stars in its central region are packed 25 times more densely than stars in Earth's region in the Milky Way. M59-UCD3 is approximately the same size as M60-UCD1 with a half-light radius , r h , of approximately 20 parsecs but is 40% more luminous with an absolute visual magnitude of approximately −14.6. This makes M59-UCD3
154-438: Is dynamical friction . The time-scales for dynamical friction on luminous (or L*) galaxies suggest that fossil groups are old, undisturbed systems that have seen little infall of L* galaxies since their initial collapse. Fossil groups are thus an important laboratory for studying the formation and evolution of galaxies and the intragroup medium in an isolated system. Fossil groups may still contain unmerged dwarf galaxies , but
176-500: The Dark Ages within the first billion years after the Big Bang . More than 20 known dwarf galaxies orbit the Milky Way, and recent observations have also led astronomers to believe the largest globular cluster in the Milky Way, Omega Centauri , is in fact the core of a dwarf galaxy with a black hole at its centre, which was at some time absorbed by the Milky Way. In astronomy ,
198-526: The Leo Ring , a cloud of hydrogen and helium around two massive galaxies in the constellation Leo . Because of their small size, dwarf galaxies have been observed being pulled toward and ripped by neighbouring spiral galaxies , resulting in stellar streams and eventually galaxy merger . There are many dwarf galaxies in the Local Group ; these small galaxies frequently orbit larger galaxies, such as
220-687: The Milky Way , the Andromeda Galaxy and the Triangulum Galaxy . A 2007 paper has suggested that many dwarf galaxies were created by galactic tides during the early evolutions of the Milky Way and Andromeda. Tidal dwarf galaxies are produced when galaxies collide and their gravitational masses interact . Streams of galactic material are pulled away from the parent galaxies and the halos of dark matter that surround them. A 2018 study suggests that some local dwarf galaxies formed extremely early, during
242-707: The Sunflower Galaxy (M63) , is located to the southeast of the M101 Group, and the NGC 5866 Group is located to the northwest. The distances to these three groups (as determined from the distances to the individual member galaxies) are similar, which suggests that the M51 Group, the M101 Group, and the NGC 5866 Group are actually part of a single large, loose, elongated group. However, most group identification methods (including those used by
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#1732855523047264-650: The Milky Way and contains over 30 billion stars, is sometimes classified as a dwarf galaxy; others consider it a full-fledged galaxy. Dwarf galaxies' formation and activity are thought to be heavily influenced by interactions with larger galaxies. Astronomers identify numerous types of dwarf galaxies, based on their shape and composition. One theory states that most galaxies, including dwarf galaxies, form in association with dark matter , or from gas that contains metals. However, NASA 's Galaxy Evolution Explorer space probe identified new dwarf galaxies forming out of gases with low metallicity . These galaxies were located in
286-672: The Nearby Galaxies Catalog, the survey of Fouque et al., the Lyons Groups of Galaxies (LGG) Catalog, and the three group lists created from the Nearby Optical Galaxy sample of Giuricin et al. Other possible members galaxies (galaxies listed in only one or two of the lists from the above references) include the irregular galaxies NGC 5238 and UGC 8508 . The M51 Group , which includes the Whirlpool Galaxy (M51) and
308-758: The hearts of rich clusters. UCDs have been found in the Virgo Cluster , Fornax Cluster , Abell 1689 , and the Coma Cluster , amongst others. In particular, an unprecedentedly large sample of ~ 100 UCDs has been found in the core region of the Virgo cluster by the Next Generation Virgo Cluster Survey team. The first ever relatively robust studies of the global properties of Virgo UCDs suggest that UCDs have distinct dynamical and structural properties from normal globular clusters. An extreme example of UCD
330-435: 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, accounting for at least 50% of the galaxies in the local universe. Groups have a mass range between those of the very large elliptical galaxies and clusters of galaxies. In
352-544: The local universe, about half of the groups exhibit diffuse X-ray emissions from their intracluster media . Those that emit X-rays appear to have early-type galaxies as members. The diffuse X-ray emissions come from zones within the inner 10–50% of the groups' virial radius, generally 50–500 kpc. There are several subtypes of groups. A compact group consists of a small number of galaxies, typically around five, in close proximity and relatively isolated from other galaxies and formations. The first compact group to be discovered
374-399: The more massive members of the group have condensed into the central galaxy. This hypothesis is supported by studies of computer simulations of cosmological volumes. The closest fossil group to the Milky Way is NGC 6482 , an elliptical galaxy at a distance of approximately 180 million light-years located in the constellation of Hercules . Proto-groups are groups that are in
396-409: The most dark matter -dominated systems known. Astronomers believe that UFDs encode valuable information about the early Universe , as all UFDs discovered so far are ancient systems that have likely formed very early on, only a few million years after the Big Bang and before the epoch of reionization . Recent theoretical work has hypothesised the existence of a population of young UFDs that form at
418-410: The process of forming new stars . The galaxies' stars are all formed at different time periods, so the galaxies have time to cool and to build up matter to form new stars. As time passes, this star formation changes the shape of the galaxies. Nearby examples include NGC 1705 , NGC 2915 , NGC 3353 and UGCA 281 . Ultra-faint dwarf galaxies (UFDs) are a class of galaxies that contain from
440-462: The process of formation. They are the smaller form of protoclusters. These contain galaxies and protogalaxies embedded in dark matter haloes that are in the process of fusing into group-formations of singular dark matter halos. Dwarf galaxies A dwarf galaxy is a small galaxy composed of about 1000 up to several billion stars , as compared to the Milky Way 's 200–400 billion stars. The Large Magellanic Cloud , which closely orbits
462-421: The references cited above) identify these three groups as separate entities. Group of galaxies The Milky Way galaxy is part of a group of galaxies called the Local Group . Groups of galaxies are the smallest aggregates of galaxies. They typically contain no more than 50 galaxies in a diameter of 1 to 2 megaparsecs (Mpc). Their mass is approximately 10 solar masses . The spread of velocities for
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#1732855523047484-481: Was Stephan's Quintet , found in 1877. Stephan's Quintet is named for a compact group of four galaxies plus an unassociated foreground galaxy. Astronomer Paul Hickson created a catalogue of such groups in 1982, the Hickson Compact Groups . Compact groups of galaxies readily show the effect of dark matter , as the visible mass is greatly less than that needed to gravitationally hold the galaxies together in
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