Boötes II (dwarf galaxy)

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]

Notes

References

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