Paper detail

How common is the Milky Way - satellite system alignment?

The highly flattened distribution of satellite galaxies in the Milky Way presents a number of puzzles. Firstly, its polar alignment stands out from the planar alignments commonly found in other galaxies. Secondly, recent proper motion measurements reveal that the orbital angular momentum of at least 3, and possibly as many as 8, of the Milky Ways satellites point (within 30 degrees) along the axis of their flattened configuration, suggesting some form of coherent motion. In this paper we use a high resolution cosmological simulation to investigate whether this pattern conflicts with the expectations of the cold dark matter model of structure formation. We find that this seemingly unlikely set up occurs often: approximately 35% of the time we find systems in which the angular momentum of 3 individual satellites point along, or close to, the short axis of the satellite distribution. In addition, in 30% of the systems we find that the net angular momentum of the 6 best aligned satellites lies within 35 degrees of the short axis of the satellite distribution, as observed for the Milky Way.