Structure of the M31 satellite system: Bayesian distances from the tip of the Red Giant Branch

The satellite system of a large galaxy represents the ideal laboratory for the study of galactic evolution. Whether that evolution has been dominated by past mergers or in situ formation, clues abound within the structure of the satellite system. This study utilizes recent photometric data obtained for the halo of M31 via the Pan-Andromeda Archaeological Survey (PAndAS), to undertake an analysis of the spatial distribution of the M31 satellite system. To do this, a new Bayesian algorithm is developed for measuring the distances to the satellites from the tip of their Red Giant Branch. The distances are obtained in the form of posterior probability distributions, which give the probability of the satellite lying at any given distance after accounting for the various spatial and photometric characteristics of the component stars. Thus robust distances are obtained for M31 and 27 of its satellite galaxies which are then transformed into three-dimensional, M31-centric positions yielding a homogenous sample of unprecedented size in any galaxy halo. A rigorous analysis of the resulting distribution is then undertaken, with the homogeneity of the sample fully exploited in characterizing the effects of data incompleteness. This analysis reveals a satellite distribution which as a whole, is roughly isothermal and no more planar than one would expect from a random distribution of equal size. A subset of 15 satellites is however found to be remarkably planar, with a root-mean-square thickness of just 12.34kpc. Of these satellites, 13 have subsequently been identified as co-rotating. This highly significant plane is all the more striking for its orientation. From the Earth we view it perfectly edge on and it is almost perpendicular to the Milky Way’s disk. Furthermore, it is roughly orthogonal to the disk-like structure commonly reported for the Milky Way’s satellite galaxies. The distribution is also found to be highly asymmetric, with the majority of satellites lying on the near side of M31. These findings point to a complex evolutionary history with possible links to that of our own galaxy.