Adwarf galaxy is a smallgalaxy composed of about 1000 up to several billionstars, as compared to theMilky Way's 200–400 billion stars.[1] TheLarge Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars,[2] 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.
Because of their small size, dwarf galaxies have been observed being pulled toward andripped by neighbouringspiral galaxies, resulting instellar streams and eventuallygalaxy merger.[6] Recent studies, like the Hyper Suprime-Cam Subaru Strategic Program, have shown that around 30-50% of dwarf galaxies that reside in low-density environments are quenched, meaning they have littlestar formation. This indicates that dwarf galaxy evolution is largely influenced by stellar oractive galactic nuclei as well as the environments of nearby massive galaxies[7].
ThePhoenix Dwarf Galaxy is a dwarf irregular galaxy, featuring younger stars in its inner regions and older ones at its outskirts.[8]
There are many dwarf galaxies in theLocal Group; these small galaxies frequently orbit larger galaxies, such as theMilky Way, theAndromeda Galaxy and theTriangulum Galaxy. A 2007 paper[9] has suggested that many dwarf galaxies were created bygalactic tides during the early evolutions of the Milky Way and Andromeda. Tidal dwarf galaxies are produced when galaxies collide and their gravitationalmasses interact. Streams of galactic material are pulled away from the parent galaxies and the halos ofdark matter that surround them.[10] A 2018 study suggests that some local dwarf galaxies formed extremely early, during theDark Ages within the first billion years after theBig Bang.[11] Additionally, recent studies also show nearby star-forming dwarf galaxies have systems that may have formed most of their stellar mass within the last billion years, indicated by blue colours, high star-formation rates, andlow metallicities[12].
More than 20 known dwarf galaxies orbit the Milky Way, and recent observations[13] have also led astronomers to believe the largestglobular cluster in the Milky Way,Omega Centauri, is in fact the core of a dwarf galaxy with ablack hole at its centre, which was at some time absorbed by the Milky Way.
Measurements of the motions of individual stars from local dwarf galaxies, such as theSculptor Dwarf Galaxy, have allowed scientists to test predictions of the standard cosmological model.[14] Detailed studies of local dwarf galaxies also provide insight into how environmental effects likeram pressure stripping shape the evolution and formation of galaxies.[15]
Inastronomy, ablue compact dwarf galaxy (BCD galaxy) is a small galaxy which contains largeclusters of young, hot, massivestars. These stars, the brightest of which are blue, cause the galaxy itself to appearblue in colour.[19] Most BCD galaxies are also classified as dwarfirregular galaxies or as dwarflenticular 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 the process offorming 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.
Ultra-faint dwarf galaxies (UFDs) are a class ofgalaxies that contain from a few hundred to one hundred thousandstars, making them the faintest galaxies in theUniverse.[24] UFDs resembleglobular clusters (GCs) in appearance but have very different properties. Unlike GCs, UFDs contain a significant amount ofdark matter and are more extended. UFDs were first discovered with the advent of digital sky surveys in 2005, in particular with theSloan Digital Sky Survey (SDSS).[25][26]
UFDs are the mostdark matter-dominated systems known.Astronomers believe that UFDs encode valuable information about the earlyUniverse, as all UFDs discovered so far are ancient systems that have likely formed very early on, only a few million years after theBig Bang and before the epoch ofreionization.[27] Recent theoretical work has hypothesised the existence of a population of young UFDs that form at a much later time than the ancient UFDs.[28] These galaxies have not been observed in ourUniverse so far.
Ultra-compact dwarf galaxies (UCD) are a class of very compact galaxies with very high stellar densities,discovered[29][30][31] in the 2000s.They are thought to be on the order of 200 light years across, containing about 100 million stars.[32] There are two principal theorized formation channels for these galaxies. The first is that these are the cores of nucleated dwarf elliptical galaxies that have been stripped of gas and outlying stars bytidal interactions, traveling through the hearts of rich clusters,[33] and the second is that these are massivestar clusters.[34] UCDs have been found in theVirgo Cluster,Fornax Cluster,Abell 1689, and theComa Cluster, amongst others.[35]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.[36] The first ever relatively robust studies of the global properties of Virgo UCDs suggest thatUCDs have distinct dynamical[37]and structural[38] properties from normal globular clusters. An extreme example of UCD isM60-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.[39][40]M59-UCD3 is approximately the same size as M60-UCD1 with ahalf-light radius, rh, of approximately 20parsecs but is 40% more luminous with anabsolute visual magnitude of approximately −14.6. This makes M59-UCD3 the second densest known galaxy.[41]Based on stellar orbital velocities, two UCD in the Virgo Cluster are claimed to havesupermassive black holes weighing 13% and 18% of the galaxies' masses.[42]
^O'Callaghan, Jonathan (22 May 2024). "Astronomers find long-missing dwarf galaxies—too many of them".Science.384 (6698): 836.doi:10.1126/science.z6r95kt.
^"Evidence for densest galaxy in nearby universe". Phys.org (Omicron Technology Ltd). 24 September 2013. Retrieved25 September 2013.What makes M60-UCD1 so remarkable is that about half of this mass is found within a radius of only about 80 light years. The density of stars is about 15,000 times greater—meaning the stars are about 25 times closer to each other—than in Earth's region of theMilky Way galaxy.
^Sandoval, Michael A.; Vo, Richard P.; Romanowsky, Aaron J.; Strader, Jay; Choi, Jieun; Jennings, Zachary G.; Conroy, Charlie; Brodie, Jean P.; Foster, Caroline; Villaume, Alexa; Norris, Mark A.; Janz, Joachim; Forbes, Duncan A. (23 July 2015). "Hiding in Plain Sight: Record-breaking Compact Stellar Systems in the Sloan Digital Sky Survey".The Astrophysical Journal.808 (1): L32.arXiv:1506.08828.Bibcode:2015ApJ...808L..32S.doi:10.1088/2041-8205/808/1/L32.S2CID55254708.
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![Dwarf galaxy UGC 685 taken by Hubble.[45]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aHubble%25E2%2580%2599s_Legacy_-_Flickr_-_europeanspaceagency.jpg)
