The vast thin plane of M31 corotating dwarfs: an additional fossil signature of the M31 merger and of its considerable impact in the whole Local Group

Abstract

The recent discovery by Ibata et al. (2013) of a vast thin disk of satellites\n(VTDS) around M31 offers a new challenge for the understanding of the Local\nGroup properties. This comes in addition to the unexpected proximity of the\nMagellanic Clouds (MCs) to the Milky Way (MW), and to another vast polar\nstructure (VPOS), which is almost perpendicular to our Galaxy disk. We find\nthat the VTDS plane is coinciding with several stellar, tidally-induced streams\nin the outskirts of M31, and, that its velocity distribution is consistent with\nthat of the Giant Stream (GS). This is suggestive of a common physical\nmechanism, likely linked to merger tidal interactions, knowing that a similar\nargument may apply to the VPOS at the MW location. Furthermore, the VTDS is\npointing towards the MW, being almost perpendicular to the MW disk, as the VPOS\nis.\n We compare these properties to the modelling of M31 as an ancient, gas-rich\nmajor merger, which has been successfully used to predict the M31 substructures\nand the GS origin. We find that without fine tuning, the induced tidal tails\nare lying in the VTDS plane, providing a single and common origin for many\nstellar streams and for the vast stellar structures surrounding both the MW and\nM31. The model also reproduces quite accurately positions and velocities of the\nVTDS dSphs. Our conjecture leads to a novel interpretation of the Local Group\npast history, as a gigantic tidal tail due to the M31 ancient merger is\nexpected to send material towards the MW, including the MCs. Such a link\nbetween M31 and the MW is expected to be quite exceptional, though it may be in\nqualitative agreement with the reported rareness of MW-MCs systems in nearby\ngalaxies.\n