Leo T Explained

Leo T Dwarf Galaxy[1]
Epoch:J2000
Type:dSph/dIrr
Dist Ly:1,365 kly (420 kpc)
Appmag V:16
Size V:2.8
Constellation Name:Leo
Names:Leo T, PGC 4713564

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] The velocity with respect to the Milky Way is around −60 km/s implying a slow infall onto the Milky Way. 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 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).[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. The metallicity of these old stars is very low at, which means that they contain 100 times less heavy elements than the Sun.[4] The observed old stars are primarily red giants, although a number of horizontal branch stars and red clump stars were also discovered. 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.[5]

Leo T contains significant amount of neutral hydrogen (HI) gas with the mass of about 280,000 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 and References

  1. NAME Leo T dSph. 2010-03-14.
  2. Irwin. V. . Belokurov. V. . Evans. N. W. . etal . Discovery of an Unusual Dwarf Galaxy in the Outskirts of the Milky Way. 2007. The Astrophysical Journal. 656. 1. L13–L16. 10.1086/512183. 2007ApJ...656L..13I. astro-ph/0701154 . 18742260 .
  3. Simon. Joshua D.. Geha, Marla. Marla Geha . The Kinematics of the Ultra-faint Milky Way Satellites: Solving the Missing Satellite Problem. 2007. The Astrophysical Journal. 670. 1. 313–331. 10.1086/521816. 2007ApJ...670..313S. 0706.0516 . 9715950.
  4. Kirby. Evan N.. Simon, Joshua D. . Geha, Marla. Marla Geha . etal . Uncovering Extremely Metal-Poor Stars in the Milky Way's Ultrafaint Dwarf Spheroidal Satellite Galaxies. 2008. The Astrophysical Journal. 685. 1. L43–L46. 10.1086/592432. 2008ApJ...685L..43K . 0807.1925 . 3185311.
  5. de Jong. J.T.A.. Harris, J. . Coleman, M.G. . etal . The Structural Properties and Star Formation History of Leo T from Deep LBT Photometry. 2008. The Astrophysical Journal. 680. 2. 1112–1119. 10.1086/587835. 2008ApJ...680.1112D. 0801.4027 . 14357361.
  6. Ryan-Weber. Emma V.. Emma Ryan-Weber. Begum, Ayesha . Oosterloo, Tom . etal . The Local Group dwarf Leo T: HI on the brink of star formation. 2008. Mon. Not. R. Astron. Soc.. 384. 2. 535–540. 10.1111/j.1365-2966.2007.12734.x. free . 2008MNRAS.384..535R. 0711.2979 . 13855611.
  7. Ricotti. Massimo. Late gas accretion on to primordial minihaloes: a model for Leo T, dark galaxies and extragalactic high-velocity clouds. 2009. Mon. Not. R. Astron. Soc.. 392. 1. L45–L49. 10.1111/j.1745-3933.2008.00586.x. free . 2009MNRAS.392L..45R. 0806.2402 . 18393415.