Editing Dwarf galaxy (section)

==Ultra-faint dwarf galaxies==
{{anchor|Ultra-faint dwarf galaxies}}

{{See also|Ultra diffuse galaxy}}

[[File:Dark Matter is a No Show in Ghostly Galaxy (gemini1801b).jpg|thumb|A composite color image by the [[Hubble Space Telescope]] of the [[NGC 1052-DF2]]]]
'''Ultra-faint dwarf''' galaxies (UFDs) are a class of [[Galaxy|galaxies]] that contain from a few hundred to one hundred thousand [[star]]s, making them the faintest galaxies in the [[Universe]].<ref>{{Cite journal|last=Simon|first=Joshua D.|date=2019-08-18|title=The Faintest Dwarf Galaxies|url=https://www.annualreviews.org/doi/10.1146/annurev-astro-091918-104453|journal=Annual Review of Astronomy and Astrophysics|language=en|volume=57|issue=1|pages=375–415|doi=10.1146/annurev-astro-091918-104453|arxiv=1901.05465 |bibcode=2019ARA&A..57..375S |s2cid=119384790 |issn=0066-4146}}</ref> UFDs resemble [[globular cluster]]s (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).<ref>{{Cite journal|last1=Willman|first1=Beth|last2=Dalcanton|first2=Julianne J.|last3=Martinez-Delgado|first3=David|last4=West|first4=Andrew A.|last5=Blanton|first5=Michael R.|last6=Hogg|first6=David W.|last7=Barentine|first7=J. C.|last8=Brewington|first8=Howard J.|last9=Harvanek|first9=Michael|last10=Kleinman|first10=S. J.|last11=Krzesinski|first11=Jurek|date=2005-06-20|title=A New Milky Way Dwarf Galaxy in Ursa Major|url=https://iopscience.iop.org/article/10.1086/431760|journal=The Astrophysical Journal|language=en|volume=626|issue=2|pages=L85–L88|doi=10.1086/431760 |arxiv=astro-ph/0503552|bibcode=2005ApJ...626L..85W |s2cid=14851943 |issn=0004-637X}}</ref><ref>{{Cite journal|last1=Willman|first1=Beth|last2=Blanton|first2=Michael R.|last3=West|first3=Andrew A.|last4=Dalcanton|first4=Julianne J.|last5=Hogg|first5=David W.|last6=Schneider|first6=Donald P.|last7=Wherry|first7=Nicholas|last8=Yanny|first8=Brian|last9=Brinkmann|first9=Jon|date=June 2005|title=A New Milky Way Companion: Unusual Globular Cluster or Extreme Dwarf Satellite?|url=https://iopscience.iop.org/article/10.1086/430214|journal=The Astronomical Journal|language=en|volume=129|issue=6|pages=2692–2700|doi=10.1086/430214 |arxiv=astro-ph/0410416|bibcode=2005AJ....129.2692W |s2cid=826898 |issn=0004-6256}}</ref>

UFDs are the most [[dark matter]]-dominated systems known. [[Astronomer]]s 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]].<ref>{{Cite journal|last1=Bovill|first1=Mia S.|last2=Ricotti|first2=Massimo|title=Pre-Reionization Fossils, Ultra-Faint Dwarfs, and the Missing Galactic Satellite Problem |date=2009-03-10|url=https://iopscience.iop.org/article/10.1088/0004-637X/693/2/1859|journal=The Astrophysical Journal|volume=693|issue=2|pages=1859–1870|doi=10.1088/0004-637X/693/2/1859|arxiv=0806.2340 |bibcode=2009ApJ...693.1859B |s2cid=14543154 |issn=0004-637X}}</ref> Recent theoretical work has hypothesised the existence of a population of young UFDs that form at a much later time than the ancient UFDs.<ref>{{Cite journal|last1=Benitez-Llambay|first1=Alejandro|last2=Fumagalli|first2=Michele|date=2021-11-01|title=The Tail of Late-forming Dwarf Galaxies in ΛCDM|journal=The Astrophysical Journal Letters|volume=921|issue=1|pages=L9|doi=10.3847/2041-8213/ac3006| arxiv=2110.08279|bibcode=2021ApJ...921L...9B |s2cid=239016084 |issn=2041-8205 |doi-access=free }}</ref>  These galaxies have not been observed in our [[Universe]] so far.