Boötes II

Bootes II or Boo II is a dwarf spheroidal galaxy situated at a distance of about 42 kpc. 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 absolute visible magnitude of about −2.7 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. Currently there is no star formation in Bootes II. The measurements have so far failed to detect any neutral hydrogen in it.

Bootes II is located only ~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. 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.