The satellite distribution of M31

The spatial distribution of the Galactic satellite system plays an important role in Galactic dynamics and cosmology, where its successful reproduction is a key test of simulations of galaxy halo formation. Here, we examine its representative nature by conducting an analysis of the three-dimensional spatial distribution of the M31 subgroup of galaxies, the next closest system to our own. We begin by a discussion of distance estimates and incompleteness concerns, before revisiting the question of membership of the M31 subgroup. We constrain this by consideration of the spatial and kinematic properties of the putative satellites. Comparison of the distribution of M31 and Galactic satellites relative to the galactic discs suggests that the Galactic system is probably modestly incomplete at low latitudes by similar or equal to 20 per cent. We find that the radial distribution of satellites around M31 is more extended than the Galactic subgroup; 50 per cent of the Galactic satellites are found within similar to 100 kpc of the Galaxy, compared to similar to 200 kpc for M31. We search for 'ghostly streams' of satellites around M31, in the same way others have done for the Galaxy, and find several, including some that contain many of the dwarf spheroidal satellites. The lack of M31-centric kinematic data, however, means that we are unable to probe whether these streams represent real physical associations. Finally, we find that the M31 satellites are asymmetrically distributed with respect to our line of sight to this object, so that the majority of its satellites are on its near side with respect to our line of sight. We quantify this result in terms of the offset between M31 and the centre of its satellite distribution, and find it to be significant at the similar to 3 sigma level. We discuss possible explanations for this finding, and suggest that many of the M31 satellites may have been accreted only relatively recently. Alternatively, this anisotropy may be related to a similar result recently reported for the 2dFGRS, which would imply that the halo of M31 is not yet virialized. Until such time as a satisfactory explanation for this finding is presented, however, our results warn against treating the M31 subgroup as complete, unbiased and relaxed.