Contents

Leo T

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 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]

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]