Binary Satellite Galaxies

Understand

Suggestions have appeared in the literature that the following five pairs of Milky Way and Andromeda satellite galaxies are gravitationally bound: Draco and Ursa Minor, Leo IV and V, Andromeda I and III, NGC 147 and 185, and the Magellanic clouds.

• Under the assumption that a given pair is gravitationally bound, the Virial theorem provides an estimate of its total mass and so its instantaneous tidal radius.
• For all of these pairs except for the Magellanic clouds the resulting total mass is 2 to 4 orders of magnitude higher than that within the half light radius.
• Furthermore in the case of each pair except for Leo IV and Leo V, the estimated tidal radius is inferior to the separation between the two satellites.