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Dwarf spheroidal galaxy
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== Discovery == Despite the radii of dSphs being much larger than those of [[globular cluster]]s, they are much more difficult to find due to their low luminosities and surface brightnesses. Dwarf spheroidal galaxies have a large range of luminosities, and known dwarf spheroidal galaxies span several orders of magnitude of luminosity.<ref name=":23">{{Cite journal|last1=Strigari|first1=Louis E.|last2=Bullock|first2=James S.|last3=Kaplinghat|first3=Manoj|last4=Simon|first4=Joshua D.|last5=Geha|first5=Marla|author5-link= Marla Geha |last6=Willman|first6=Beth|last7=Walker|first7=Matthew G.|date=2008-08-28|title=A common mass scale for satellite galaxies of the Milky Way|journal=Nature|language=en|volume=454|issue=7208|pages=1096β1097|doi=10.1038/nature07222|pmid=18756252|issn=0028-0836|arxiv = 0808.3772 |bibcode = 2008Natur.454.1096S |s2cid=4373541 }}</ref> Their luminosities are so low that [[Ursa Minor]], [[Carina (constellation)|Carina]], and [[Draco (constellation)|Draco]], the known dwarf spheroidal galaxies with the lowest luminosities, have mass-to-light ratios (M/L) greater than that of the Milky Way.<ref name=":03">{{Cite book|title=Galaxies in the Universe|last1=Sparke|first1=L.S.|author1-link= Linda Sparke |last2=Gallagher|first2=J.S. III|publisher=[[Cambridge University Press]]|year=2016|isbn=978-0-521-67186-6|location=United Kingdom|pages=162β165}}</ref> Dwarf spheroidals also have little to no gas with no obvious signs of recent star formation.<ref>{{Cite journal|title=NASA/ADS Search|language=en|bibcode = 1994A&ARv...6...67F|last1=Ferguson |first1=Henry C. |last2=Binggeli |first2=Bruno |journal=Astronomy and Astrophysics Review |year=1994 |volume=6 |issue=1β2 |page=67 |doi=10.1007/BF01208252 |arxiv=astro-ph/9409079 |s2cid=18879556 }}</ref><ref>{{Cite journal|last=McConnachie|first=Alan W.|date=2012-06-05|bibcode=2012AJ....144....4M|journal=The Astronomical Journal|language=en|volume=144|issue=1|pages=4|doi=10.1088/0004-6256/144/1/4|issn=0004-6256|arxiv=1204.1562|title=The Observed Properties of Dwarf Galaxies in and Around the Local Group|s2cid=118515618 }}</ref> Within the [[Local Group]], dSphs are primarily found near the [[Milky Way]] and [[Andromeda Galaxy|M31]].<ref>{{Cite journal|title=NASA/ADS Search|language=en|bibcode = 1998ARA&A..36..435M|last1=Mateo |first1=Mario L. |journal=Annual Review of Astronomy and Astrophysics |year=1998 |volume=36 |pages=435β506 |doi=10.1146/annurev.astro.36.1.435 |arxiv=astro-ph/9810070 |s2cid=119333888 }}</ref><ref>{{Cite journal|last=K.|first=Grebel, E.|date=1998|title=Star Formation Histories of Local Group Dwarf Galaxies|bibcode=1998HiA....11..125G|journal=Highlights of Astronomy|language=en|volume=11|pages=125β126|doi=10.1017/S1539299600020190 |arxiv=astro-ph/9806191}}</ref> The first dwarf spheroidal galaxies discovered were Sculptor and Fornax in 1938.<ref name=":03" /> The [[Sloan Digital Sky Survey]] has resulted in the discovery of 11 more dSph galaxies as of 2007<ref name="simponandGeha20074">{{cite journal |last1=Simon |first1=Josh |last2=Geha |first2=Marla |author2-link= Marla Geha |date=November 2007 |title=The kinematics of the ultra-faint Milky Way satellites: Solving the missing satellite problem |journal=The Astrophysical Journal |volume=670 |issue=1 |pages=313β331 |arxiv=0706.0516 |bibcode=2007ApJ...670..313S |doi=10.1086/521816|s2cid=9715950 }}</ref> By 2015, many more ultra-faint dSphs were discovered, all satellites of the Milky Way.<ref name="Koposov20154">{{cite journal |author1=Koposov, Sergey E. |author2=Belokurov, Vasily |author3=Torrealba, Gabriel |author4=Evans, N. Wyn |date=10 March 2015 |title=Beasts of the southern wild: Discovery of a large number of ultra-faint satellites in the vicinity of the Magellanic clouds |journal=The Astrophysical Journal |volume=805 |issue=2 |pages=130 |arxiv=1503.02079 |bibcode=2015ApJ...805..130K |doi=10.1088/0004-637X/805/2/130|s2cid=118267222 }}</ref> Nine potentially new dSphs were discovered in the [[The Dark Energy Survey|Dark Energy Survey]] in 2015.<ref name=":13">{{Cite journal |last1=Bonnivard |first1=V. |last2=Combet |first2=C. |last3=Daniel |first3=M. |last4=Funk |first4=S. |last5=Geringer-Sameth |first5=A. |last6=Hinton |first6=J.A. |last7=Maurin |first7=D. |last8=Read |first8=J.I. |last9=Sarkar |first9=S. |display-authors=6 |year=2015 |title=Dark matter annihilation and decay in dwarf spheroidal galaxies: The classical and ultrafaint dSphs |journal=Monthly Notices of the Royal Astronomical Society |volume=453 |issue=1 |pages=849β867 |arxiv = 1504.02048 |bibcode = 2015MNRAS.453..849B |doi = 10.1093/mnras/stv1601 |doi-access=free }}</ref> Each dSph is named after constellations they are discovered in, such as the [[Sagittarius Dwarf Spheroidal Galaxy|Sagittarius dwarf spheroidal galaxy]], all of which consist of stars generally much older than 1β2 Gyr that formed over the span of many gigayears.<ref name=":03" /> For example, 98% of the stars in the Carina dwarf spheroidal galaxy are older than 2 Gyr, formed over the course of three bursts around 3, 7 and 13 Gyr ago.<ref name=":03" /> The stars in Carina have also been found to be metal-poor.<ref>{{Cite journal |last1=Bono |first1=G. |last2=Stetson |first2=P.B. |last3=Walker |first3=A.R. |last4=Monelli |first4=M. |last5=Fabrizio |first5=M. |last6=Pietrinferni |first6=A. |last7=Brocato |first7=E. |last8=Buonanno |first8=R. |last9=Caputo |first9=F. |display-authors=6 |date=2010-01-01 |title=On the stellar content of the Carina dwarf spheroidal galaxy |url=http://stacks.iop.org/1538-3873/122/i=892/a=651 |journal=Publications of the Astronomical Society of the Pacific |language=en |volume=122 |issue=892 |pages=651 |doi=10.1086/653590 |issn=1538-3873 |arxiv = 1004.2559 |bibcode = 2010PASP..122..651B |s2cid=119301603 }}</ref> This is unlike star clusters because, while star clusters have stars which formed more or less the same time, dwarf spheroidal galaxies experience multiple bursts of star formation.<ref name=":03"/>
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