Galaxy Pairs in the Local Group

TL;DR

This study finds that about 30% of bright dwarf satellites in the Local Group are in physical pairs, a much higher rate than predicted by models, suggesting new insights into galaxy formation.

Contribution

It reveals a high frequency of dwarf galaxy pairs in the Local Group, challenging existing models and indicating potential new clues to galaxy formation processes.

Findings

~30% of satellites are in pairs of comparable luminosity.

Pairs are closer than expected by chance and have similar velocities.

Models predict fewer than 4% of satellites form such pairs.

Abstract

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.