Galaxy Pairs in the Local Group
  Current models of galaxy formation predict that galaxy pairs of comparable
magnitudes should become increasingly rare with decreasing luminosity. This
seems at odds with the relatively high frequency of pairings among dwarf
galaxies in the Local Group. We use literature data to show that ~30% of all
satellites of the Milky Way and Andromeda galaxies brighter than M_V=-8 are
found in likely physical pairs of comparable luminosity. Besides the previously
recognised pairings of the Magellanic Clouds and of NGC 147/NGC 185, other
candidate pairs include the Ursa Minor and Draco dwarf spheroidals, as well as
the And I/And III satellites of M31. These pairs are much closer than expected
by chance if the radial and angular distributions of satellites were
uncorrelated; in addition, they have very similar line-of-sight velocities and
luminosities that differ by less than three magnitudes. In contrast, the same
criteria pair fewer than 4% of satellites in N-body/semi-analytic models that
match the radial distribution and luminosity function of Local Group
satellites. If confirmed in studies of larger samples, the high frequency of
dwarf galaxy pairings may provide interesting clues to the formation of faint
galaxies in the current cosmological paradigm.
