Boötes II (dwarf galaxy)
Encyclopedia
Bootes II or Boo II is a dwarf spheroidal galaxy
situated in the Bootes constellation and discovered in 2007 in the data obtained by Sloan Digital Sky Survey
. The galaxy is located at the distance of about 42 kpc from the Sun and moves towards the Sun with the speed of 120 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 51 pc.
Bootes II is one of the smallest and faintest satellites of the Milky Way
—its integrated luminosity is about 1,000 times that of the Sun (absolute visible magnitude of about −2.7), which is much lower than the luminosity of the majority of globular cluster
s. However the mass of the galaxy is substantial corresponding to the mass to light ratio
of more than 100.
The stellar population of Bootes II consists mainly of moderately old stars formed 10–12 billion years ago. The metallicity of these old stars is low at , which means that they contain 80 times less heavy elements than the Sun. Currently there is no star formation in Bootes II. The measurements have so far failed to detect any neutral hydrogen in it—the upper limit is only 86 solar masses.
Bootes II is located only 1.5 degrees (~1.6 kpc) away from another dwarf galaxy—Boötes I, although they are unlikely to be physically associated because they move in opposite directions relative to the Milky Way. Their relative velocity—about 200 km/s is too high. It is more likely associated with the Sagittarius Stream
and, therefore, with the Sagittarius Dwarf Elliptical Galaxy
(SagDEG). Bootes II may be either a satellite galaxy of SagDED or one of its star clusters torn from the main galaxy 4–7 billion years ago.
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 Bootes constellation and discovered in 2007 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 42 kpc from the Sun and moves towards the Sun with the speed of 120 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 51 pc.
Bootes II 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 1,000 times that of the Sun (absolute visible magnitude of about −2.7), which is much lower than the luminosity of the majority of 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...
s. However the mass of the galaxy is substantial corresponding to the 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...
of more than 100.
The stellar population of Bootes II consists mainly of moderately old stars formed 10–12 billion years ago. The metallicity of these old stars is low at , which means that they contain 80 times less heavy elements than the Sun. Currently there is no star formation in Bootes II. The measurements have so far failed to detect any neutral hydrogen in it—the upper limit is only 86 solar masses.
Bootes II is located only 1.5 degrees (~1.6 kpc) away from another dwarf galaxy—Boötes I, although they are unlikely to be physically associated because they move in opposite directions relative to the Milky Way. Their relative velocity—about 200 km/s is too high. It is more likely associated with the Sagittarius Stream
Field of Streams
The Field of Streams is a patch of sky where several stellar streams are visible and crisscross. The analysis of the streams of the zone determined that the dark matter configuration of the Milky Way Galaxy in the inner halo is round, at variance with expectations that it would be oblate.It was...
and, therefore, with the Sagittarius Dwarf Elliptical Galaxy
Sagittarius Dwarf Elliptical Galaxy
The Sagittarius Dwarf Elliptical Galaxy is an elliptical loop-shaped satellite galaxy of the Milky Way Galaxy. It consists of four globular clusters, the main cluster being discovered in 1994...
(SagDEG). Bootes II may be either a satellite galaxy of SagDED or one of its star clusters torn from the main galaxy 4–7 billion years ago.