A dwarf galaxy is a small galaxy composed of about 100 million up to several billion stars, a small number compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which closely orbits 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.
Current 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.
Local dwarf galaxiesEdit
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 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 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.
- Elliptical galaxy: dwarf elliptical galaxy (dE)
- Dwarf spheroidal galaxy (dSph): Once a subtype of dwarf ellipticals, now regarded as a distinct type
- Irregular galaxy: dwarf irregular galaxy (dIrr)
- Spiral galaxy: dwarf spiral galaxy (dS) 
- Magellanic type dwarfs
- Blue compact dwarf galaxies (see section below)
- Ultra-compact dwarf galaxies (see section below)
Blue compact dwarf galaxiesEdit
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. 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.
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 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 is M60-UCD1, about 54 million light years away, which contains approximately 200 million solar masses within a 160 light year radius; its central region packs in stars about 25 times closer together 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, rh, of approximately 20 parsecs but is 40% more luminous with an absolute visual magnitude of approximately −14.6. This makes M59-UCD3 the densest known galaxy. 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.
- Aquarius Dwarf
- Canis Major Dwarf Galaxy
- Crater 2 dwarf
- Eridanus II
- Henize 2-10
- I Zwicky 18
- IC 10
- Large Magellanic Cloud
- NGC 1569
- NGC 1705
- NGC 2915
- NGC 3353
- Pegasus Dwarf Irregular Galaxy
- Phoenix Dwarf
- Sagittarius Dwarf Elliptical Galaxy
- Sagittarius Dwarf Irregular Galaxy
- Sculptor Dwarf Galaxy
- Sculptor Dwarf Irregular Galaxy
- Sextans A
- Sextans Dwarf Spheroidal
- Small Magellanic Cloud
- Tucana Dwarf
- Ursa Minor Dwarf
- Willman 1
- Carina Dwarf
- Draco Dwarf
- Fornax Dwarf
- Leo I
- Leo II
- "An irregular island". www.spacetelescope.org. Retrieved 25 August 2016.
- "New Recipe For Dwarf Galaxies: Start With Leftover Gas". Science Daily. 19 February 2009. Retrieved 29 July 2015.
- Jaggard, V. (9 September 2010). "Pictures: New Proof Spiral Galaxies Eat, Digest Dwarfs". National Geographic Society. Retrieved 11 February 2012.
- "Hubble Sizes up a Dwarf Galaxy". ESA/Hubble. 24 October 2011. Retrieved 25 October 2011.
- Metz, M.; Kroupa, P. (2007). "Dwarf-spheroidal satellites: are they of tidal origin?". Monthly Notices of the Royal Astronomical Society. 376 (1): 387–392. arXiv:astro-ph/0701289. Bibcode:2007MNRAS.376..387M. doi:10.1111/j.1365-2966.2007.11438.x.
- "New Recipe for Dwarf Galaxies: Start with Leftover Gas". Newswise.com. 18 February 2009. Retrieved 20 February 2009.
- Rincon, Paul (16 August 2018). "Earliest galaxies found 'on our cosmic doorstep'". BBC News. Retrieved 17 August 2018.
- Noyola, E.; Gebhardt, K.; Bergmann, M. (2008). "Gemini and Hubble Space Telescope Evidence for an Intermediate-Mass Black Hole in ω Centauri". The Astrophysical Journal. 676 (2): 1008–1015. arXiv:0801.2782. Bibcode:2008ApJ...676.1008N. doi:10.1086/529002.
- "True blue". ESA/Hubble. 15 June 2015. Retrieved 15 June 2015.
- Schombert, J. M.; Pildis, R. A.; Eder, J. A.; Oelmer, A., Jr. (1995). "Dwarf Spirals". The Astronomical Journal. 110: 2067–2074. Bibcode:1995AJ....110.2067S. doi:10.1086/117669.
- "An intriguing young-looking dwarf galaxy". ESA/Hubble. 16 March 2015. Retrieved 21 March 2015.
- "WISE Discovers Baby Galaxies in the Nearby Universe". WISE. 2 September 2011. Retrieved 3 September 2011.
- López-Sánchez, Á. R.; Koribalski, B.; van Eymeren, J.; Esteban, C.; Popping, A.; Hibbard, J. (2010). "The environment of nearby Blue Compact Dwarf Galaxies". ASP Conference Series. 421: 65. arXiv:0909.5500. Bibcode:2010ASPC..421...65L.
- Papaderos, P. (7 May 2010). "Blue Compact Dwarf Galaxies" (PDF). Centro de Astrofísica da Universidade do Porto.
- Noeske, K.; Papaderos, P.; Cairos, L. M. (2003). "New insights to the photometric structure of Blue Compact Dwarf Galaxies from deep Near-Infrared Studies" (PDF). Göttingen Observatory. Archived from the original (PDF) on 15 August 2011.
- Meurer, G. R.; Mackie, G.; Carignan, C. (1994). "Optical observations of NGC 2915: A nearby blue compact dwarf galaxy". The Astronomical Journal. 107 (6): 2021–2035. Bibcode:1994AJ....107.2021M. doi:10.1086/117013.
- Hilker, M.; Infante, L.; Vieira, G.; Kissler-Patig, M.; Richtler, T. (1999). "The central region of the Fornax cluster. II. Spectroscopy and radial velocities of member and background galaxies". Astronomy and Astrophysics Supplement. 134: 75–86. arXiv:astro-ph/9807144. Bibcode:1999A&AS..134...75H. doi:10.1051/aas:1999434.
- Drinkwater, M. J.; Jones, J. B.; Gregg, M. D.; Phillipps, S. (2000). "Compact Stellar Systems in the Fornax Cluster: Super-massive Star Clusters or Extremely Compact Dwarf Galaxies?". Publications of the Astronomical Society of Australia. 17: 227–233. arXiv:astro-ph/0002003. Bibcode:2000PASA...17..227D. doi:10.1071/AS00034.
- Smith, Deborah (29 May 2003). "Star search finds millions masquerading as one". Sydney Morning Herald. p. 5. ISSN 0312-6315.
- Anglo-Australian Observatory Astronomers discover dozens of mini-galaxies 0100 AEST Friday 2 April 2004.
- Stelios Kazantzidis; Ben Moore; Lucio Mayer (2004). "Galaxies and Overmerging: What Does it Take to Destroy a Satellite Galaxy?". ASP Conference Series. 327: 155. arXiv:astro-ph/0307362. Bibcode:2004ASPC..327..155K.
- Mieske; Infante; Benitez; Coe; Blakeslee; Zekser; Ford; Broadhurst; et al. (2004). "Ultra Compact Dwarf galaxies in Abell 1689: a photometric study with the ACS". The Astronomical Journal. 128 (4): 1529–1540. arXiv:astro-ph/0406613. Bibcode:2004AJ....128.1529M. doi:10.1086/423701.
- Zhang, Hong-Xin; et al. (March 2015). "The Next Generation Virgo Cluster Survey. VI: The Kinematics of Ultracompact Dwarfs and Globular Clusters in M87". Astrophysical Journal. 802: 30. arXiv:1501.03167. Bibcode:2015ApJ...802...30Z. doi:10.1088/0004-637X/802/1/30.
- Liu, Chengze; et al. (November 2015). "The Next Generation Virgo Cluster Survey. X: Properties of Ultracompact Dwarfs in The M87, M49 and M60 Regions". Astrophysical Journal. 812: 34. arXiv:1508.07334. Bibcode:2015ApJ...812...34L. doi:10.1088/0004-637X/812/1/34.
- Strader, Jay; Seth, Anil C.; Forbes, Duncan A.; Fabbiano, Giuseppina; et al. (August 2013). "The Densest Galaxy". Astrophysical Journal Letters. 775 (1): L6. arXiv:1307.7707. Bibcode:2013ApJ...775L...6S. doi:10.1088/2041-8205/775/1/L6. Retrieved 25 September 2013.
"Evidence for densest galaxy in nearby universe". Phys.org (Omicron Technology Ltd). 24 September 2013. Retrieved 25 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.
- 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.
- 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. (2017). "Detection of Supermassive Black Holes in Two Virgo Ultracompact Dwarf Galaxies". Astrophysical Journal. 839: 72. arXiv:1703.09221. Bibcode:2017ApJ...839...72A. doi:10.3847/1538-4357/aa6972.
- "A stubborn dwarf galaxy". Retrieved 6 July 2016.
- "A galaxy of deception". Retrieved 29 September 2014.
- "Hubble's Legacy". www.spacetelescope.org. Retrieved 2 September 2019.