Right ascension 13 58 00 Apparent magnitude (V) 15.8 ± 0.5 Apparent size (V) 8.0+2.2
−2.8′ | Declination +12° 51′ 00″ Type dSph Constellation Boötes | |
Distance 136 ± 7 kly (42 ± 2 kpc) Similar Leo IV, Willman 1, Ursa Major II Dwarf, Hercules, Segue 1 | ||
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 clusters. 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 [Fe/H]=−1.8, 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 SagDEG or one of its star clusters torn from the main galaxy 4–7 billion years ago.