Boötes III

Bootes III is an overdensity in the Milky Way's halo, which may be a disrupted dwarf spheroidal galaxy. It is situated at a distance of about 46 kpc and has an elongated shape (axis ratio of 2:1) with the radius of about 0.5 kpc. The large size and an irregular shape may indicate that Bootes III in a transitional phase between a gravitationally bound galaxy and completely unbound system.

Boots III is one of the smallest and faintest satellites of the Milky Way — its absolute visible magnitude of about −5.8 is much lower than the luminosity of many globular clusters. The mass of Bootes III is difficult to estimate because the galaxy is in process of being disrupted. In this case the velocity dispersion of its stars is not related to its mass.

The stellar population of Bootes III consists mainly of moderately old stars formed more than 12 billion years ago. The metallicity of these old stars is low at [Fe/H]=−2.1 ± 0.2. Bootes III may be the source of stars of the Styx stream in the galactic halo.