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 3-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 disks suggests that the Galactic system is probably modestly incomplete at low latitudes by ≃ 20 %. We find that the radial distribution of satellites around M31 is more extended than the Galactic subgroup; 50 % of the Galactic satellites are found within ∼ 100kpc of the Galaxy, compared to ∼ 200kpc 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 which contain many