A Dynamical Model for the Orbit of the Andromeda Galaxy M31 and the Origin of the Local Group of Galaxies

We propose a new model for the origin and evolution of the Local Group of Galaxies (LGG) that naturally explains the formation of the Magellanic Clouds and their large orbital angular momenta around the Galaxy. The basic idea is that an off-center hydrodynamical collision occurred some 10Gyr ago between the primordial Andromeda galaxy (M31) and a similar Galaxy, and compressed the halo gas to form the LGG dwarf galaxies, including the Magellanic Clouds. New-born dwarf galaxies can be expected to locate on the orbital plane of these two massive galaxies. We reexamined the two-dimensional sky distribution of the LGG members, and confirmed an early idea that they align along two similar great circles. The planes of these circles are approximately normal to the line joining the present position of the Sun and the galactic center. We made a distribution map of these objects, and found a well-defined plane of finite thickness. Thus we could determine the orbital elements of M31 relative to the Galaxy by reproducing the well-studied dynamics of the LMC and the SMC around the Galaxy. The expected proper motion of M31 is (µl ,µ b )=( 38±16 µasyr −1 , −49 ±5 µasyr −1 ).