View source for Leo I (dwarf galaxy)

{{Short description| Dwarf spheroidal Galaxy in the constellation Leo }}
{{Infobox Galaxy
| image = Ugc5470.jpg
| image_size = 250px
| caption = Leo I appears as a faint patch to the right of the bright star, [[Regulus]].
| name = Leo I
| epoch = [[Epoch (astronomy)#Julian years and J2000|J2000]]
| type = E;dSph<ref name="ned">{{cite web
  | title=NASA/IPAC Extragalactic Database
  | work=Results for Leo I
  | url=http://nedwww.ipac.caltech.edu/
  | accessdate=2006-11-29 }}</ref>
| ra = {{RA|10|08|27.4}}<ref name="ned" />
| dec = {{DEC|+12|18|27}}<ref name="ned" />
| dist_ly = [[1 E21 m|820 ± 70]] [[light-year|kly]] (250 ± 20 [[parsec|kpc]])<ref name="karachentsevetal2004">{{cite journal
  |author1=I. D. Karachentsev |author2=V. E. Karachentseva |author3=W. K. Hutchmeier |author4=D. I. Makarov | title=A Catalog of Neighboring Galaxies
  | journal=Astronomical Journal
  | date=2004
  | volume=127
  | issue=4
  | pages=2031–2068
  | bibcode=2004AJ....127.2031K  | doi=10.1086/382905
| doi-access=free}}</ref><ref name="Karachentsevetal2006">{{cite journal
  | author=Karachentsev, I. D.
  | author2=Kashibadze, O. G.
  | title=Masses of the local group and of the M81 group estimated from distortions in the local velocity field
  | journal=Astrophysics
  | date=2006
  | volume=49
  | issue=1
  | pages=3–18
  | bibcode=2006Ap.....49....3K   | doi=10.1007/s10511-006-0002-6
| s2cid=120973010
 }}</ref>
| z = 285 ± 2 km/[[second|s]]<ref name="ned" />
| appmag_v = 11.2<ref name="ned" />
| absmag_v = &minus;12.0<ref name=RuizLara>{{cite journal|doi=10.1093/mnras/staa3871|title=Dissecting the stellar content of Leo I: A dwarf irregular caught in transition|year=2021|last1=Ruiz-Lara|first1=T.|last2=Gallart|first2=C.|last3=Monelli|first3=M.|last4=Fritz|first4=T. K.|last5=Battaglia|first5=G.|last6=Cassisi|first6=S.|last7=Aznar|first7=M Luis|last8=Russo Cabrera|first8=A. V.|last9=Rodríguez-Martín|first9=I.|last10=Salazar-González|first10=J. J.|journal=Monthly Notices of the Royal Astronomical Society|volume=501|issue=3|pages=3962–3980|doi-access=free |arxiv=2012.07863}}</ref>
| size_v = 9.8{{prime}} × 7.4{{prime}}<ref name="ned" />
| constellation name = [[Leo (constellation)|Leo]]
| notes = [[Milky Way]] satellite
| names = [[Uppsala General Catalogue|UGC]] 5470,<ref name="ned" /> [[Principal Galaxies Catalogue|PGC]] 29488,<ref name="ned" /> [[David Dunlap Observatory Catalogue|DDO]] 74,<ref name="ned" /> A1006,<ref name="ned" /> Harrington-Wilson #1,<ref name="ned" /> Regulus Dwarf<ref name="ned" />
|mass=(2.0 ± 1.0) × 10<sup>7</sup>
|size=2000 ly<ref name=Pultarova/>
}}

'''Leo I''' is a [[dwarf spheroidal galaxy]] in the [[constellation]] [[Leo (constellation)|Leo]]. At about 820,000 [[light-year]]s distant, it is a member of the [[Local Group]] of galaxies and is thought to be one of the most distant satellites of the [[Milky Way]] galaxy. It was discovered in 1950 by [[Albert George Wilson]] on [[photographic plate]]s of the [[National Geographic Society – Palomar Observatory Sky Survey]], which were taken with the 48-inch [[Schmidt camera]] at [[Palomar Observatory]].<ref name="SEDS">{{cite web
 | title=Leo I
 | work=SEDS Messier Database
 | url=http://messier.seds.org/xtra/ngc/leo1.html
 | accessdate=2006-05-15
}}</ref><ref name="Bergh">{{cite book
 | first=Sidney
 | last=Van den Bergh
 | date=2000
 | title=Galaxies of the Local Group
 | edition=1st
 | publisher=[[Cambridge University Press]]
 | isbn=0-521-65181-6
 | pages=243–245
}}</ref>

==Visibility==

Leo I is located only 12 [[minute of arc|arc minutes]] from [[Regulus]], the brightest star in the constellation. For that reason, the galaxy is sometimes called the ''Regulus Dwarf''. Scattered light from the star makes studying the galaxy more difficult, and it was not until the 1990s that it was detected visually.<ref name="SEDS"/><ref name="Bergh"/>

The proximity of [[Regulus]] and the low surface brightness make it a real challenge to observe. Medium-sized amateur telescopes (15&nbsp;cm or more) and a dark sky appear to be required for a sighting. But some reports of April 2013 tell that one observer with an 11&nbsp;cm mini Dobson and even a refractor as small as 7&nbsp;cm f/10 has sighted Leo I under very dark sky conditions.<ref name="sighting">{{cite web
 |title       = Faint Fuzzy Observations
 |url         = http://freescruz.com/~4cygni/faintfuzzies.blogspot.com/april-skywatcher.html
 |accessdate  = 2014-03-24
 |url-status     = dead
 |archiveurl  = https://archive.today/20140324193057/http://freescruz.com/~4cygni/faintfuzzies.blogspot.com/april-skywatcher.html
 |archivedate = 2014-03-24
}}</ref>

==Mass==

The measurement of [[radial velocity|radial velocities]] of some bright [[red giant]]s in Leo I have made possible to measure its mass. It was found to be at least (2.0 ± 1.0) × 10<sup>7</sup> {{Solar mass|link=y}}. The results are not conclusive, and do not exclude or confirm the existence of a large [[dark matter]] halo around the galaxy. However, it seems to be certain that the galaxy does not rotate.<ref name="Bergh"/>

A kinematic study of Leo I could not place much constraints on dark matter, but suggested the presence of a [[black hole]] of three million [[solar mass]]es in the center of the galaxy. This would be significant, as it would be the first time this has been done with a dwarf spheroidal galaxy.<ref>{{cite journal|doi=10.3847/1538-4357/ac0c79|title=Dynamical Analysis of the Dark Matter and Central Black Hole Mass in the Dwarf Spheroidal Leo I|year=2021|last1=Bustamante-Rosell|first1=M. J.|last2=Noyola|first2=Eva|last3=Gebhardt|first3=Karl|last4=Fabricius|first4=Maximilian H.|last5=Mazzalay|first5=Ximena|last6=Thomas|first6=Jens|last7=Zeimann|first7=Greg|journal=The Astrophysical Journal|volume=921|issue=2|page=107|arxiv=2111.04770|bibcode=2021ApJ...921..107B|s2cid=243798608 |doi-access=free }}</ref> A black hole of three million solar masses is comparable to the mass of the Milky Way's black hole, [[Sagittarius A*]].<ref name=Pultarova>{{Cite web|last=Pultarova|first=Tereza|date=2021-12-03|title=Giant black hole inside a tiny satellite galaxy of our Milky Way defies explanation|url=https://www.space.com/giant-black-hole-in-dwarf-galaxy-defies-explanation|access-date=2021-12-03|website=Space.com|language=en-US}}</ref> However, another study could not confirm this, suggesting at most an intermediate-mass black hole of a few 10<sup>5</sup> solar masses.<ref>{{cite journal | doi=10.1051/0004-6361/202449620 | arxiv=2403.14784 | bibcode=2024A&A...684L..19P | title=The central black hole in the dwarf spheroidal galaxy Leo I: Not supermassive, at most an intermediate-mass candidate | date=2024 | last1=Pascale | first1=R. | last2=Nipoti | first2=C. | last3=Calura | first3=F. | last4=Della Croce | first4=A. | journal=Astronomy & Astrophysics | volume=684 | pages=L19 }}</ref>

It has been suggested that Leo I is a tidal debris stream in the outer [[galactic halo|halo]] of the Milky Way. This hypothesis has not been confirmed, however.<ref name="Bergh"/>

==Star formation==
Typical to a dwarf galaxy, the [[metallicity]] of Leo I is very low, only one percent that of the [[Sun]]. Gallart et al. (1999) deduce from [[Hubble Space Telescope]] observations that the galaxy experienced a major increase (accounting for 70% to 80% of its population) in its star formation rate between 6 [[annum|Ga]] and 2 Ga (billion years ago). There is no significant evidence of any stars that are more than 10 Ga old. About 1 Ga ago, star formation in Leo I appears to have dropped suddenly to an almost negligible rate, roughly coinciding with its latest periastron passage of the Milky Way. [[Ram pressure]] stripping would have removed its gas, decreasing its star formation rate.<ref name=RuizLara/> Some low-level activity may have continued until 200-500 Ma. Therefore, it is thought to be the youngest dwarf spheroidal satellite galaxy of the Milky Way. In addition, the galaxy may be embedded in a cloud of ionized gas with a mass similar to that of the whole galaxy.<ref name="Bergh"/><ref name="Bergh2000">{{Cite journal
 | last1 = van den Bergh
 | first1 = Sidney
 | authorlink1 = Sidney van den Bergh
 | title = Updated Information on the Local Group
 | series = 1sss
 | date = April 2000
 | journal = The Publications of the Astronomical Society of the Pacific
 | volume = 112
 | issue = 770
 | bibcode = 2000PASP..112..529V
 | pages = 529–536
 | doi = 10.1086/316548
 |arxiv = astro-ph/0001040 | s2cid = 1805423
 }}</ref>

==References==

{{reflist}}

==External links==
{{commons category}}
* {{WikiSky|Leo I}}
* {{APOD |date=3 October 1999 |title=Leo I}}
* [http://spider.seds.org/spider/LG/leo1.html SEDS page on Leo I]

{{Milky Way}}
{{Sky|10|08|27.4|+|12|18|27|820000}}

[[Category:Dwarf spheroidal galaxies]]
[[Category:Local Group]]
[[Category:Milky Way Subgroup]]
[[Category:Leo (constellation)]]
[[Category:UGC objects|05470]]
[[Category:Principal Galaxies Catalogue objects|29488]]
[[Category:Astronomical objects discovered in 1950|?]]