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Leo I
Leo I
	Leo I appears as a faint patch to the right of the bright star, Regulus.
Observation data (J2000 epoch)
Constellation	Leo
Right ascension	10h 08m 27.4s
Declination	+12° 18′ 27″
Redshift	285 ± 2 km/s
Distance	820 ± 70 kly (250 ± 20 kpc)
Apparent magnitude (V)	11.2
Absolute magnitude (V)	−12.0
Characteristics
Type	E;dSph
Mass	(2.0 ± 1.0) × 107 M☉
Size	2000 ly
Apparent size (V)	9.8′ × 7.4′
Notable features	Milky Way satellite
Other designations
	UGC 5470, PGC 29488, DDO 74, A1006, Harrington-Wilson #1, Regulus Dwarf

Leo I is a dwarf spheroidal galaxy in the constellation Leo. At about 820,000 light-years 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 plates of the National Geographic Society – Palomar Observatory Sky Survey, which were taken with the 48-inch Schmidt camera at Palomar Observatory.^[6]^[7]

Visibility

Leo I is located only 12 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.^[6]^[7]

The proximity of Regulus and the low surface brightness make it a real challenge to observe. Medium-sized amateur telescopes (15 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 cm mini Dobson and even a refractor as small as 7 cm f/10 has sighted Leo I under very dark sky conditions.^[8]

Mass

The measurement of radial velocities of some bright red giants in Leo I have made possible to measure its mass. It was found to be at least (2.0 ± 1.0) × 10⁷ M_☉. 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.^[7]

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 masses 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.^[9] A black hole of three million solar masses is comparable to the mass of the Milky Way's black hole, Sagittarius A*.^[5] However, another study could not confirm this, suggesting at most an intermediate-mass black hole of a few 10⁵ solar masses.^[10]

It has been suggested that Leo I is a tidal debris stream in the outer halo of the Milky Way. This hypothesis has not been confirmed, however.^[7]

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 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.^[4] 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.^[7]^[11]

References

^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l "NASA/IPAC Extragalactic Database". Results for Leo I. Retrieved 2006-11-29.
^ I. D. Karachentsev; V. E. Karachentseva; W. K. Hutchmeier; D. I. Makarov (2004). "A Catalog of Neighboring Galaxies". Astronomical Journal. 127 (4): 2031–2068. Bibcode:2004AJ....127.2031K. doi:10.1086/382905.
^ Karachentsev, I. D.; Kashibadze, O. G. (2006). "Masses of the local group and of the M81 group estimated from distortions in the local velocity field". Astrophysics. 49 (1): 3–18. Bibcode:2006Ap.....49....3K. doi:10.1007/s10511-006-0002-6. S2CID 120973010.
^ ^a ^b Ruiz-Lara, T.; Gallart, C.; Monelli, M.; Fritz, T. K.; Battaglia, G.; Cassisi, S.; Aznar, M Luis; Russo Cabrera, A. V.; Rodríguez-Martín, I.; Salazar-González, J. J. (2021). "Dissecting the stellar content of Leo I: A dwarf irregular caught in transition". Monthly Notices of the Royal Astronomical Society. 501 (3): 3962–3980. arXiv:2012.07863. doi:10.1093/mnras/staa3871.
^ ^a ^b Pultarova, Tereza (2021-12-03). "Giant black hole inside a tiny satellite galaxy of our Milky Way defies explanation". Space.com. Retrieved 2021-12-03.
^ ^a ^b "Leo I". SEDS Messier Database. Retrieved 2006-05-15.
^ ^a ^b ^c ^d ^e Van den Bergh, Sidney (2000). Galaxies of the Local Group (1st ed.). Cambridge University Press. pp. 243–245. ISBN 0-521-65181-6.
^ "Faint Fuzzy Observations". Archived from the original on 2014-03-24. Retrieved 2014-03-24.
^ Bustamante-Rosell, M. J.; Noyola, Eva; Gebhardt, Karl; Fabricius, Maximilian H.; Mazzalay, Ximena; Thomas, Jens; Zeimann, Greg (2021). "Dynamical Analysis of the Dark Matter and Central Black Hole Mass in the Dwarf Spheroidal Leo I". The Astrophysical Journal. 921 (2): 107. arXiv:2111.04770. Bibcode:2021ApJ...921..107B. doi:10.3847/1538-4357/ac0c79. S2CID 243798608.
^ Pascale, R.; Nipoti, C.; Calura, F.; Della Croce, A. (2024). "The central black hole in the dwarf spheroidal galaxy Leo I: Not supermassive, at most an intermediate-mass candidate". Astronomy & Astrophysics. 684: L19. arXiv:2403.14784. Bibcode:2024A&A...684L..19P. doi:10.1051/0004-6361/202449620.
^ van den Bergh, Sidney (April 2000). "Updated Information on the Local Group". The Publications of the Astronomical Society of the Pacific. 1sss. 112 (770): 529–536. arXiv:astro-ph/0001040. Bibcode:2000PASP..112..529V. doi:10.1086/316548. S2CID 1805423.