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) | |
| Constellation | Leo |
| Right ascension | 09h 34m 53.4s[1] |
| Declination | +17° 03′ 05″[1] |
| Distance | 1,365 kly (420 kpc)[2] |
| Apparent magnitude (V) | 16[2] |
| Characteristics | |
| Type | dSph/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 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.[clarification needed] 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
References
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