Leo T (dwarf galaxy)
Encyclopedia
Leo T is a dwarf galaxy
situated in the Leo constellation and discovered in 2006 in the data obtained by Sloan Digital Sky Survey
. 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. 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 dwarf irregular galaxies
(dIrr). Its half-light radius
is about 180 pc.
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). However, its mass is about 8 million solar mass
es, 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 the dark matter
.
s, 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 to the center of Leo T than the old population. Currently there is no star formation in this galaxy.
Leo T contains significant amount of neutral hydrogen (HI) gas with the mass of about 2.8 solar masses, which is three times more than the mass of the stars in this galaxy. The gas includes two main components: cool gas in the center of the galaxy with the temperature of about 500 K and warm gas distributed throughout Leo T with the temperature of 6,000 K. The density of this gas is, however, not enough on average for the 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.
Leo T galaxy may have formed when a small dark matter halo
started accretion of gas some time after the re-ionization epoch. Latter this gas gave birth to the first generation of old stars.
Dwarf galaxy
A dwarf galaxy is a small galaxy composed of up to several billion stars, a small number compared to our own Milky Way's 200-400 billion stars...
situated in the Leo constellation and discovered in 2006 in the data obtained by Sloan Digital Sky Survey
Sloan Digital Sky Survey
The Sloan Digital Sky Survey or SDSS is a major multi-filter imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project was named after the Alfred P...
. 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. 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
Dwarf spheroidal galaxy
Dwarf spheroidal galaxy is a term in astronomy applied to low luminosity galaxies that are companions to the Milky Way and to the similar systems that are companions to the Andromeda Galaxy M31...
(dSph) and dwarf dwarf irregular galaxies
Irregular galaxy
An irregular galaxy is a galaxy that does not have a distinct regular shape, like a spiral or an elliptical galaxy. The shape of an irregular galaxy is uncommon – they do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, with neither a...
(dIrr). Its half-light radius
Effective radius
The effective radius of a galaxy is the radius at which one half of the total light of the system is emitted interior to this radius. This assumes the galaxy is circularly symmetric...
is about 180 pc.
Leo T is one of the smallest and faintest galaxies in the Local Group
Local Group
The Local Group is the group of galaxies that includes Earth's galaxy, the Milky Way. The group comprises more than 30 galaxies , with its gravitational center located somewhere between the Milky Way and the Andromeda Galaxy...
—its integrated luminosity is about 40,000 times that of the Sun (absolute visible magnitude of about −7.1). However, its mass is about 8 million solar mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
es, which means that Leo's mass to light ratio
Mass to light ratio
In astrophysics and physical cosmology the mass to light ratio, normally designated with the symbol \Upsilon is the quotient between the total mass of a spatial volume and its luminosity...
is around 140. A high mass to light ratio implies that Leo T is dominated by the dark matter
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...
.
Neutral hydrogen and star formation
The stellar population of Leo T consists of both old and young stars. 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. The observed old stars are primarily red giantRed giant
A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower...
s, although a number of horizontal branch
Horizontal branch
The horizontal branch is a stage of stellar evolution which immediately follows the red giant branch in stars whose masses are similar to the Sun's...
stars and red clump
Red clump
The red clump is a feature in the Hertzsprung-Russell diagram of stars. The red clump is considered the metal-rich counterpart to the horizontal branch. Stars in this part of the Hertzsprung-Russell diagram are sometimes called clump giants...
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 to the center of Leo T than the old population. Currently there is no star formation in this galaxy.
Leo T contains significant amount of neutral hydrogen (HI) gas with the mass of about 2.8 solar masses, which is three times more than the mass of the stars in this galaxy. The gas includes two main components: cool gas in the center of the galaxy with the temperature of about 500 K and warm gas distributed throughout Leo T with the temperature of 6,000 K. The density of this gas is, however, not enough on average for the 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.
Leo T galaxy may have formed when a small dark matter halo
Dark matter halo
A dark matter halo is a hypothetical component of a galaxy, which extends beyond the edge of the visible galaxy and dominates the total mass. Since they consist of dark matter, halos cannot be observed directly, but their existence is inferred through their effects on the motions of stars and gas...
started accretion of gas some time after the re-ionization epoch. Latter this gas gave birth to the first generation of old stars.