Galaxy pairs in the Local Group

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. 1 We define all virial quantities as those corresponding to a sphere of mean density equal to 200 times the critical density for closure. Furthermore, we shall hereafter refer to the stellar mass of a galaxy as "galaxy mass", for brevity.

NGC 147, NGC 185 and CassII: a genetic approach to orbital properties, star formation and tidal debris

“…By only considering NGC 147 and NGC 185, van den Bergh (1998) firstly explored this possibility, concluding that the mass, morphology and velocity of these galaxies strongly suggest that they form a bound pair. This initial hypothesis is corroborated by the recent results presented by Fattahi et al (2013), who identified NGC 147 and NGC 185 as likely bound galaxies. Based on these works, we use the genetic algorithm to select orbits such that NGC 147 and NGC 185 are bound at least once in the past 8 Gyr.…”

“…In addition to the potential association of the Magellanic Clouds, confirmed by recent dynamical studies of their interactions (Besla et al 2012;Diaz & Bekki 2012;Guglielmo, Lewis & BlandHawthorn 2014), some of the M31 satellites are possible members of bound pairs, like And I/And III and NGC 147/NGC 185 (Fattahi et al 2013). The suggestion that the later form a bound pair was already proposed by van den Bergh (1998), who argued that due to the proximity in positions, distances and velocities, these satellites are likely to be associated.…”

“…Such associations present strong spatial and kinematic correlations and appear to have high mass-to-light ratios. Fattahi et al (2013) showed that ∼ 30 per cent of Local Group satellites, brighter than Mv = −8, are likely in bound pairs. This analysis also reveals interesting galaxy pair candidates.…”

The extended structure of the dwarf irregular galaxy Sagittarius

“…Both a rotation amplitude lower than the H  velocity dispersion and chaotic motions induced by astrophysical mechanisms (e.g., star formation, gas in-/outflows) probably contribute to masking the underlying rotation signal, if any. Also the merging with another dwarf may have played a role in producing the disturbed H  morphology and velocity field (see, e.g., Deason et al 2014;Fattahi et al 2013, and references therein).…”

Sweating the small stuff: simulating dwarf galaxies, ultra-faint dwarf galaxies, and their own tiny satellites

“…This suggests that one way to search for ultra-faint galaxies might be as satellites of known dwarf galaxies. Associations and pairs of satellite galaxies in the Local Group have been observed for some time (Tully et al 2006;D'Onghia & Lake 2008;Ibata et al 2013;Fattahi et al 2013), and several Milky Way satellites are suspected to host their own satellites (Pace et al 2014;Deason et al 2014).…”