Binary Satellite Galaxies
  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. Therefore all or almost all of these systems are
not gravitationally bound. We note several possible explanations for the
proximities and similar radial velocities of the satellites in each pair, for
example they may have condensed from the same infalling structure or they may
be bound by a nongravitational interaction between dark matter and an extremely
light particle.
