Leo IV (dwarf galaxy)
Encyclopedia
Leo IV is a dwarf spheroidal 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 160 kpc from the Sun and moves away from the Sun with the velocity of about 130 km/s. It is classified as a dwarf spheroidal galaxy
(dSph) meaning that it has an approximately round shape with the half-light radius of about 130 pc.
Leo IV is one of the smallest and faintest satellites of the Milky Way
—its integrated luminosity is about 15,000 times that of the Sun (absolute visible magnitude of ), which is much lower than the luminosity of a typical globular cluster
. However, its mass is about 1.5 million solar mass
es, which means that Leo's mass to light ratio
is around 150. A high mass to light ratio implies that Leo IV is dominated by the dark matter
.
The stellar population of Leo IV consists mainly of old stars formed more than 12 billion years ago. The metallicity of these old stars is also very low at , which means that they contain 400 times less heavy elements than the Sun. The observed stars were primarily red giant
s, although a number of Horizontal branch
stars including three variable RR Lyrae
stars were also discovered. The stars of Leo IV were probably among the first stars to form in the Universe
. Nevertheless, the detailed study of the stellar population revealed the presence of a small number of much younger stars with the age of about 2 billion years or less. This discovery points to a complicated star formation
history of this galaxy. Currently there is no star formation in Leo IV. The measurements have so far failed to detect any neutral hydrogen in it—the upper limit is just 600 solar masses.
In 2008, another galaxy called Leo V
was discovered in the vicinity of Leo IV. The former is located 20 kpc further from the Milky Way than the latter and 3 degrees (~ 10 kpc) away from it. These two galaxies may be physically associated with each other.
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...
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 160 kpc from the Sun and moves away from the Sun with the velocity of about 130 km/s. It is classified as a dwarf spheroidal galaxy
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) meaning that it has an approximately round shape with the half-light radius of about 130 pc.
Leo IV is one of the smallest and faintest satellites of the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
—its integrated luminosity is about 15,000 times that of the Sun (absolute visible magnitude of ), which is much lower than the luminosity of a typical globular cluster
Globular cluster
A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The name of this category of star cluster is...
. However, its mass is about 1.5 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 150. A high mass to light ratio implies that Leo IV 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...
.
The stellar population of Leo IV consists mainly of old stars formed more than 12 billion years ago. The metallicity of these old stars is also very low at , which means that they contain 400 times less heavy elements than the Sun. The observed stars were primarily red giant
Red 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 including three variable RR Lyrae
RR Lyrae
RR Lyrae is a variable star in the Lyra constellation. It is the prototype of the RR Lyrae variable class of stars. It has a period of about 13 hours, and oscillates between apparent magnitudes 7 and 8. Its variable nature was discovered by the Scottish astronomer Williamina Fleming at Harvard...
stars were also discovered. The stars of Leo IV were probably among the first stars to form in the Universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...
. Nevertheless, the detailed study of the stellar population revealed the presence of a small number of much younger stars with the age of about 2 billion years or less. This discovery points to a complicated star formation
Star formation
Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy star formation includes the study of the interstellar medium and giant molecular clouds as precursors to the star formation process and the study of young...
history of this galaxy. Currently there is no star formation in Leo IV. The measurements have so far failed to detect any neutral hydrogen in it—the upper limit is just 600 solar masses.
In 2008, another galaxy called Leo V
Leo V (dwarf galaxy)
Leo V is a dwarf spheroidal galaxy situated in the Leo constellation and discovered in 2007 in the data obtained by Sloan Digital Sky Survey. The galaxy is located at the distance of about 180 kpc from the Sun and moves away from the Sun with the velocity of about 173 km/s...
was discovered in the vicinity of Leo IV. The former is located 20 kpc further from the Milky Way than the latter and 3 degrees (~ 10 kpc) away from it. These two galaxies may be physically associated with each other.