Boötes II (dwarf galaxy)

Boötes II (dwarf galaxy)

Bootes II Dwarf Galaxy[1]
Observation data (J2000 epoch)
Constellation Bootes
Right ascension 13h 58m 00s[1]
Declination 12° 51′ 00″[1]
Distance 136 ± 7 kly (42 ± 2 kpc)[2][3]
Type dSph[4]
Apparent dimensions (V) 8.0+2.2
Apparent magnitude (V) 15.8 ± 0.5[4]
Other designations
Boo II[2]
See also: Galaxy, List of galaxies

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.[4] 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.[4][5] 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.[2][note 1]

Bootes II is one of the smallest and faintest satellites[note 2] 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 clusters.[2] However the mass of the galaxy is substantial corresponding to the mass to light ratio of more than 100.[3]

The stellar population of Bootes II consists mainly of moderately old stars formed 10–12 billion years ago.[5] The metallicity of these old stars is low at [Fe/H]=−1.8, which means that they contain 80 times less heavy elements than the Sun.[3][5] 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.[6]

Bootes II is located only 1.5 degrees (~1.6 kpc) away from another dwarf galaxy—Boötes I,[4] 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 SagDEG or one of its star clusters torn from the main galaxy 4–7 billion years ago.[5]