Leo T (dwarf galaxy)

Leo T is a dwarf galaxy situated in the Leo constellation and discovered in 2006 in the data obtained by Sloan Digital Sky Survey.[2] The galaxy is located at the distance of about 420 kpc from the Sun and moves away from the Sun with the velocity of about 35 km/s.[2][3] The velocity with respect to the Milky Way is around −60 km/s implying a slow infall onto the Milky Way.[3] Leo T is classified as a transitional object ('T' in the name) between dwarf spheroidal galaxies (dSph) and dwarf irregular galaxies (dIrr). Its half-light radius is about 180 pc.[2]

Leo T Dwarf Galaxy[1]
Observation data (J2000 epoch)
ConstellationLeo
Right ascension 09h 34m 53.4s[1]
Declination+17° 03′ 05″[1]
Distance1,365 kly (420 kpc)[2]
Apparent magnitude (V)16[2]
Characteristics
TypedSph/dIrr
Apparent size (V)2.8′[2]
Other designations
Leo T,[1] PGC 4713564

Leo T is one of the smallest and faintest galaxies in the Local Group—its integrated luminosity is about 40,000 times that of the Sun (absolute visible magnitude of about −7.1).[note 1][2] However, its mass is about 8 million solar masses, which means that Leo's mass to light ratio is around 140. A high mass to light ratio implies that Leo T is dominated by dark matter.[3]

Neutral hydrogen and star formation

The stellar population of Leo T consists of both old and young stars.[2] The old stars probably formed from 12 to 6 billion years ago.[4] The metallicity of these old stars is very low at [Fe/H] ≈ −2.02 ± 0.54,[note 2] which means that they contain 100 times less heavy elements than the Sun.[5] The observed old stars are primarily red giants, although a number of horizontal branch stars and red clump stars were also discovered.[4] After a pause star formation activity resumed about 1 billion years ago resulting in a generation of blue young stars. These young stars, which comprise only about 10% of all stellar mass, appear to be more concentrated at the center of Leo T than the old population. Currently there is no star formation in this galaxy.[4]

Leo T contains significant amount of neutral hydrogen (HI) gas with the mass of about 2.8 × 105 solar masses, which is three times more than the mass of the stars in this galaxy.[6] The gas includes two main components: cool gas in the center of the galaxy with a temperature of about 500 K and warm gas distributed throughout Leo T with a temperature of 6,000 K.[6] The density of this gas is, however, not enough on average for star formation, which indicates that local processes have a role. Still the presence of hydrogen gas implies that in the future the galaxy will begin forming stars again.[6]

Leo T galaxy may have formed when a small dark matter halo started accretion of gas some time after the reionization epoch. Later this gas gave birth to the first generation of old stars.[7]

Notes
  1. From another source the absolute magnitude is about −8.0.[4]
  2. Other sources report metallicity of about −1.5.[4]

References
  1. "NAME Leo T dSph". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2010-03-14.
  2. Irwin, V.; Belokurov, V.; Evans, N. W.; et al. (2007). "Discovery of an Unusual Dwarf Galaxy in the Outskirts of the Milky Way". The Astrophysical Journal. 656 (1): L13–L16. arXiv:astro-ph/0701154. Bibcode:2007ApJ...656L..13I. doi:10.1086/512183.
  3. Simon, Joshua D.; Geha, Marla (2007). "The Kinematics of the Ultra-faint Milky Way Satellites: Solving the Missing Satellite Problem". The Astrophysical Journal. 670 (1): 313–331. arXiv:0706.0516. Bibcode:2007ApJ...670..313S. doi:10.1086/521816.
  4. de Jong, J.T.A.; Harris, J.; Coleman, M.G.; et al. (2008). "The Structural Properties and Star Formation History of Leo T from Deep LBT Photometry". The Astrophysical Journal. 680 (2): 1112–1119. arXiv:0801.4027. Bibcode:2008ApJ...680.1112D. doi:10.1086/587835.
  5. Kirby, Evan N.; Simon, Joshua D.; Geha, Marla; et al. (2008). "Uncovering Extremely Metal-Poor Stars in the Milky Way's Ultrafaint Dwarf Spheroidal Satellite Galaxies". The Astrophysical Journal. 685 (1): L43–L46. arXiv:0807.1925. Bibcode:2008ApJ...685L..43K. doi:10.1086/592432.
  6. Ryan-Weber, Emma V.; Begum, Ayesha; Oosterloo, Tom; et al. (2008). "The Local Group dwarf Leo T: HI on the brink of star formation". Mon. Not. R. Astron. Soc. 384 (2): 535–540. arXiv:0711.2979. Bibcode:2008MNRAS.384..535R. doi:10.1111/j.1365-2966.2007.12734.x.
  7. Ricotti, Massimo (2009). "Late gas accretion on to primordial minihaloes: a model for Leo T, dark galaxies and extragalactic high-velocity clouds". Mon. Not. R. Astron. Soc. 392 (1): L45–L49. arXiv:0806.2402. Bibcode:2009MNRAS.392L..45R. doi:10.1111/j.1745-3933.2008.00586.x.
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