Leo IV is a dwarf spheroidal galaxy situated in the Leo

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the e ...

, discovered in 2006 in the data obtained by the

Sloan Digital Sky Survey The Sloan Digital Sky Survey or SDSS is a major multi-spectral 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 began in 2000 a ...

. The galaxy is located at the distance of about 160 kpc from the Sun and moves away from the Sun with the

velocity Velocity is the directional speed of an object in motion as an indication of its rate of change in position as observed from a particular frame of reference and as measured by a particular standard of time (e.g. northbound). Velocity is a ...

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 The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...

; its integrated luminosity is about 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 The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass ...

es, which means that Leo's

mass to light ratio In astrophysics and physical cosmology the mass-to-light ratio, normally designated with the Greek letter upsilon, , is the quotient between the total mass of a spatial volume (typically on the scales of a galaxy or a cluster) and its luminosity ...

is around 150. A high mass to light ratio implies that Leo IV is dominated by the

dark matter Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not ab ...

. 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 A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around or ...

s, although a number of

Horizontal branch The horizontal branch (HB) is a stage of stellar evolution that immediately follows the red-giant branch in stars whose masses are similar to the Sun's. Horizontal-branch stars are powered by helium fusion in the core (via the triple-alpha process) ...

stars including three

RR Lyrae variable RR Lyrae variables are periodic variable stars, commonly found in globular clusters. They are used as standard candles to measure (extra) galactic distances, assisting with the cosmic distance ladder. This class is named after the prototype and ...

stars were also discovered. The stars of Leo IV were probably among the first stars to form in the

Universe The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the universe. Acc ...

. 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 is the process by which dense regions within molecular clouds in The "medium" is present further soon.-->interstellar space

history of this galaxy. Currently there is no star formation in Leo IV. The measurements have so far failed to detect any

neutral hydrogen The hydrogen line, 21 centimeter line, or H I line is the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. This electromagnetic radiation has a precise frequency of , w ...

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.

Notes

References

{{Milky Way Dwarf spheroidal galaxies 4713561 Leo (constellation) Local Group Milky Way Subgroup ?