THE THREE-DIMENSIONAL STRUCTURE OF THE M31 SATELLITE SYSTEM; STRONG EVIDENCE FOR AN INHOMOGENEOUS DISTRIBUTION OF SATELLITES

Abstract

We undertake an investigation into the spatial structure of the M31 satellite\nsystem utilizing the distance distributions presented in a previous\npublication. These distances make use of the unique combination of depth and\nspatial coverage of the Pan-Andromeda Archaeological Survey (PAndAS) to provide\na large, homogeneous sample consisting of 27 of M31's satellites, as well as\nM31 itself. We find that the satellite distribution, when viewed as a whole, is\nno more planar than one would expect from a random distribution of equal size.\nA disk consisting of 15 of the satellites is however found to be highly\nsignificant, and strikingly thin, with a root-mean-square thickness of just\n$12.34^{+0.75}_{-0.43}$ kpc. This disk is oriented approximately edge on with\nrespect to the Milky Way and almost perpendicular to the Milky Way disk. It is\nalso roughly orthogonal to the disk like structure regularly reported for the\nMilky Way satellite system and in close alignment with M31's Giant Stellar\nStream. A similar analysis of the asymmetry of the M31 satellite distribution\nfinds that it is also significantly larger than one would expect from a random\ndistribution. In particular, it is remarkable that 20 of the 27 satellites most\nlikely lie on the Milky Way side of the galaxy, with the asymmetry being most\npronounced within the satellite subset forming the aforementioned disk. This\nlopsidedness is all the more intriguing in light of the apparent orthogonality\nobserved between the satellite disk structures of the Milky Way and M31.\n