The Three-Dimensional Structure of the M31 Satellite System; Strong
  Evidence for an Inhomogeneous Distribution of Satellites
  We undertake an investigation into the spatial structure of the M31 satellite
system utilizing the distance distributions presented in a previous
publication. These distances make use of the unique combination of depth and
spatial coverage of the Pan-Andromeda Archaeological Survey (PAndAS) to provide
a large, homogeneous sample consisting of 27 of M31's satellites, as well as
M31 itself. We find that the satellite distribution, when viewed as a whole, is
no more planar than one would expect from a random distribution of equal size.
A disk consisting of 15 of the satellites is however found to be highly
significant, and strikingly thin, with a root-mean-square thickness of just
$12.34^{+0.75}_{-0.43}$ kpc. This disk is oriented approximately edge on with
respect to the Milky Way and almost perpendicular to the Milky Way disk. It is
also roughly orthogonal to the disk like structure regularly reported for the
Milky Way satellite system and in close alignment with M31's Giant Stellar
Stream. A similar analysis of the asymmetry of the M31 satellite distribution
finds that it is also significantly larger than one would expect from a random
distribution. In particular, it is remarkable that 20 of the 27 satellites most
likely lie on the Milky Way side of the galaxy, with the asymmetry being most
pronounced within the satellite subset forming the aforementioned disk. This
lopsidedness is all the more intriguing in light of the apparent orthogonality
observed between the satellite disk structures of the Milky Way and M31.
