Dwarf galaxy explained

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.[1] The Large 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.

Formation

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 the Leo Ring, a cloud of hydrogen and helium around two massive galaxies in the constellation Leo.[3]

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.[4]

Local dwarf galaxies

There are many dwarf galaxies in the Local Group; these small galaxies frequently orbit larger galaxies, such as the Milky Way, the Andromeda Galaxy and the Triangulum Galaxy. A 2007 paper[5] 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.[6] A 2018 study suggests that some local dwarf galaxies formed extremely early, during the Dark Ages within the first billion years after the Big Bang.[7]

More than 20 known dwarf galaxies orbit the Milky Way, and recent observations[8] 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.

Common types

Blue compact dwarf galaxies

In astronomy, 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.[10] 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 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.[11] [12] [13] [14]

Ultra-faint dwarf galaxies

Ultra-faint dwarf galaxies (UFDs) are a class of galaxies that contain from a few hundred to one hundred thousand stars, making them the faintest galaxies in the Universe.[15] 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).[16] [17]

UFDs are 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.[18] Recent theoretical work has hypothesised the existence of a population of young UFDs that form at a much later time than the ancient UFDs.[19] These galaxies have not been observed in our Universe so far.

Ultra-compact dwarfs

Ultra-compact dwarf galaxies (UCD) are a class of very compact galaxies with very high stellar densities,discovered[20] [21] [22] in the 2000s.They are thought to be on the order of 200 light years across, containing about 100 million stars.[23] 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 the hearts of rich clusters.[24] UCDs have been found in the Virgo Cluster, Fornax Cluster, Abell 1689, and the Coma Cluster, amongst others.[25] 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.[26] The first ever relatively robust studies of the global properties of Virgo UCDs suggest thatUCDs have distinct dynamical[27] and structural[28] properties from normal globular clusters. An extreme example of UCD 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.[29] [30] M59-UCD3 is approximately the same size as M60-UCD1 with a half-light radius, rh, of approximately 20 parsecs but is 40% more luminous with an absolute visual magnitude of approximately −14.6. This makes M59-UCD3 the second densest known galaxy.[31] Based on stellar orbital velocities, two UCD in the Virgo Cluster are claimed to have supermassive black holes weighing 13% and 18% of the galaxies' masses.[32]

Partial list

External links

Notes and References

  1. Web site: The Milky Way Galaxy . 2023-06-22 . www.messier.seds.org.
  2. Web site: 2021-12-08 . The Magellanic Clouds, our galactic neighbors . 2023-06-22 . earthsky.org . en-US.
  3. Web site: 19 February 2009 . New Recipe For Dwarf Galaxies: Start With Leftover Gas . . 2015-07-29.
  4. Web site: Jaggard . V. . 9 September 2010 . Pictures: New Proof Spiral Galaxies Eat, Digest Dwarfs . https://web.archive.org/web/20100912161830/http://news.nationalgeographic.com/news/2010/09/photogalleries/100909-spiral-galaxies-eat-dwarf-space-science-pictures/ . dead . 12 September 2010 . . 2012-02-11.
  5. Metz . M. . Kroupa . P. . 2007 . Dwarf-spheroidal satellites: are they of tidal origin? . . 376 . 1 . 387–392 . astro-ph/0701289 . 2007MNRAS.376..387M . 10.1111/j.1365-2966.2007.11438.x. free . 16426005 .
  6. Web site: 18 February 2009 . New Recipe for Dwarf Galaxies: Start with Leftover Gas . Newswise.com . 2009-02-20.
  7. News: Rincon . Paul . Earliest galaxies found 'on our cosmic doorstep' . 17 August 2018 . BBC News . 16 August 2018.
  8. Noyola . E. . Gebhardt . K. . Bergmann . M. . 2008 . Gemini and Hubble Space Telescope Evidence for an Intermediate-Mass Black Hole in ω Centauri . . 676 . 2 . 1008–1015 . 0801.2782 . 2008ApJ...676.1008N . 10.1086/529002. 208867075 .
  9. Schombert . J.M. . Pildis . R.A. . Eder . J.A. . Oelmer . A. Jr. . 1995 . Dwarf Spirals . . 110 . 2067–2074 . 1995AJ....110.2067S . 10.1086/117669. free.
  10. Web site: 2 September 2011 . WISE Discovers Baby Galaxies in the Nearby Universe . . 2011-09-03.
  11. López-Sánchez . Á. R. . Koribalski . B. . van Eymeren . J. . Esteban . C. . Popping . A. . Hibbard . J. . 2010 . The environment of nearby Blue Compact Dwarf Galaxies . . 421 . 65 . 0909.5500 . 2010ASPC..421...65L.
  12. Web site: Papaderos . P. . 7 May 2010 . Blue Compact Dwarf Galaxies . Centro de Astrofísica da Universidade do Porto.
  13. Web site: Noeske . K. . Papaderos . P. . Cairos . L. M. . 2003 . New insights to the photometric structure of Blue Compact Dwarf Galaxies from deep Near-Infrared Studies . https://web.archive.org/web/20110815201613/http://estallidos.iac.es/estallidos/workshops/workshopI/charlas/KNoeske.pdf . 2011-08-15 . Göttingen Observatory.
  14. Meurer . G. R. . Mackie . G. . Carignan . C. . 1994 . Optical observations of NGC 2915: A nearby blue compact dwarf galaxy . . 107 . 6 . 2021 - 2035 . 1994AJ....107.2021M . 10.1086/117013.
  15. Simon. Joshua D.. 2019-08-18. The Faintest Dwarf Galaxies. Annual Review of Astronomy and Astrophysics. en. 57. 1. 375–415. 10.1146/annurev-astro-091918-104453. 1901.05465 . 2019ARA&A..57..375S . 119384790 . 0066-4146.
  16. Willman. Beth. Dalcanton. Julianne J.. Martinez-Delgado. David. West. Andrew A.. Blanton. Michael R.. Hogg. David W.. Barentine. J. C.. Brewington. Howard J.. Harvanek. Michael. Kleinman. S. J.. Krzesinski. Jurek. 2005-06-20. A New Milky Way Dwarf Galaxy in Ursa Major. The Astrophysical Journal. en. 626. 2. L85–L88. 10.1086/431760 . astro-ph/0503552. 2005ApJ...626L..85W . 14851943 . 0004-637X.
  17. Willman. Beth. Blanton. Michael R.. West. Andrew A.. Dalcanton. Julianne J.. Hogg. David W.. Schneider. Donald P.. Wherry. Nicholas. Yanny. Brian. Brinkmann. Jon. June 2005. A New Milky Way Companion: Unusual Globular Cluster or Extreme Dwarf Satellite?. The Astronomical Journal. en. 129. 6. 2692–2700. 10.1086/430214 . astro-ph/0410416. 2005AJ....129.2692W . 826898 . 0004-6256.
  18. Bovill. Mia S.. Ricotti. Massimo. Pre-Reionization Fossils, Ultra-Faint Dwarfs, and the Missing Galactic Satellite Problem . 2009-03-10. The Astrophysical Journal. 693. 2. 1859–1870. 10.1088/0004-637X/693/2/1859. 0806.2340 . 2009ApJ...693.1859B . 14543154 . 0004-637X.
  19. Benitez-Llambay. Alejandro. Fumagalli. Michele. 2021-11-01. The Tail of Late-forming Dwarf Galaxies in ΛCDM. The Astrophysical Journal Letters. 921. 1. L9. 10.3847/2041-8213/ac3006. 2110.08279. 2021ApJ...921L...9B . 239016084 . 2041-8205 . free .
  20. Hilker, M. . Infante, L. . Vieira, G. . Kissler-Patig, M. . Richtler, T. . The central region of the Fornax cluster. II. Spectroscopy and radial velocities of member and background galaxies . . 134 . 1999 . 75–86 . 10.1051/aas:1999434 . 1999A&AS..134...75H . astro-ph/9807144. 17710039 .
  21. Drinkwater, M. J. . Jones, J. B. . Gregg, M. D. . Phillipps, S. . Compact Stellar Systems in the Fornax Cluster: Super-massive Star Clusters or Extremely Compact Dwarf Galaxies? . . 17 . 3 . 2000 . 227–233 . 10.1071/AS00034 . 2000PASA...17..227D . astro-ph/0002003. 13161406 .
  22. News: Smith. Deborah. Star search finds millions masquerading as one. Sydney Morning Herald. 29 May 2003. 0312-6315. 5.
  23. Anglo-Australian Observatory Astronomers discover dozens of mini-galaxies 0100 AEST Friday 2 April 2004.
  24. astro-ph/0307362 . Stelios Kazantzidis . Ben Moore . Lucio Mayer . Galaxies and Overmerging: What Does it Take to Destroy a Satellite Galaxy? . 2004 . . 327 . 155 . 2004ASPC..327..155K.
  25. Mieske . Infante . Benitez . Coe . Blakeslee . Zekser . Ford . Broadhurst . Illingworth . Ultra Compact Dwarf galaxies in Abell 1689: a photometric study with the ACS . 10.1086/423701 . 2004 . . 128 . 4 . 1529–1540 . astro-ph/0406613 . 2004AJ....128.1529M . 8 . Hartig. G. F. . Clampin. M. . Ardila. D. R. . Bartko. F. . Bouwens. R. J. . Brown. R. A. . Burrows. C. J. . Cheng. E. S. . Cross. N. J. G. . Feldman. P. D. . Franx. M. . Golimowski. D. A. . Goto. T. . Gronwall. C. . Holden. B. . Homeier. N. . Kimble. R. A. . Krist. J. E. . Lesser. M. P. . Martel. A. R. . Menanteau. F.. 15575071 .
  26. Web site: The Next Generation Virgo Cluster Survey. 2023-08-20.
  27. The Next Generation Virgo Cluster Survey. VI: The Kinematics of Ultracompact Dwarfs and Globular Clusters in M87 . . March 2015 . Zhang. Hong-Xin. etal. 10.1088/0004-637X/802/1/30 . 1501.03167 . 2015ApJ...802...30Z . 802 . 1 . 30. 73517961 .
  28. The Next Generation Virgo Cluster Survey. X: Properties of Ultracompact Dwarfs in The M87, M49 and M60 Regions . . November 2015 . Liu. Chengze. etal. 10.1088/0004-637X/812/1/34 . 1508.07334 . 2015ApJ...812...34L . 812 . 1 . 34. 35610312 .
  29. The Densest Galaxy . . August 2013 . 775 . 1 . L6 . 10.1088/2041-8205/775/1/L6 . Strader. Jay . 4 . Seth. Anil C. . Forbes. Duncan A. . Fabbiano . Giuseppina . Romanowsky. Aaron J. . Brodie. Jean P. . Conroy . Charlie . Caldwell. Nelson . Pota . Vincenzo . Usher. Christopher . Arnold. Jacob A. . 1307.7707 . 2013ApJ...775L...6S. 52207639 .
  30. Web site: Evidence for densest galaxy in nearby universe. Phys.org (Omicron Technology Ltd). 25 September 2013. 24 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 the Milky Way galaxy..
  31. 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. . Hiding in Plain Sight: Record-breaking Compact Stellar Systems in the Sloan Digital Sky Survey . . 23 July 2015 . 808 . 1 . L32 . 10.1088/2041-8205/808/1/L32 . 1506.08828 . 2015ApJ...808L..32S . 55254708 .
  32. Ahn, C. P. . Seth, A. C. . den Brok, M. . Strader, J. . Baumgardt, H. . van den Bosch, R. . Chilingarian, I. . Frank, M. . Hilker, M. . McDermid, R. . Mieske, S. . Romanowsky, A. J. . Spitler, L. . Brodie, J. . Neumayer, N. . Walsh, J. L. . Detection of Supermassive Black Holes in Two Virgo Ultracompact Dwarf Galaxies . . 839 . 2 . 2017 . 72 . 10.3847/1538-4357/aa6972 . 2017ApJ...839...72A . 1703.09221. 55131811 . free .

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