DOES M31 RESULT FROM AN ANCIENT MAJOR MERGER?

The M31 haunted halo is likely associated with a rich merger history, currently assumed to be caused by multiple minor mergers. Here we use the GADGET2 simulation code to test whether M31 could have experienced a major merger in its past history. Our results indicate that a (3 +/- 0.5):1 gaseous-rich merger with r(pericenter) = 25 +/- 5 kpc and a polar orbit can explain many properties of M31 and of its halo. The interaction and fusion may have begun 8.75 +/- 0.35 and 5.5 +/- 0.5 Gyr ago, respectively. Observed fractions of the bulge and the thin and thick disks can be retrieved for a star formation history that is almost quiescent before the fusion. This also accords well with the observed relative fractions of intermediate age and old stars in both the thick disk and the Giant Stream. In this model, the Giant Stream is caused by returning stars from a tidal tail which contains material previously stripped from the satellite prior to the fusion. These returning stars are trapped into elliptical orbits or loops for long periods of time which can reach a Hubble time, and belong to a plane that is 45 degrees offset from the M31 disk position angle. Because these streams of stars are permanently fed by new infalling stars with high energy from the tidal tail, we predict large loops which scale rather well with the features recently discovered in the M31 outskirts. We demonstrate that a single merger could explain first-order (intensity and size), morphological, and kinematical properties of the disk, thick disk, bulge, and streams in the halo of M31, as well as the observed distribution of stellar ages, and perhaps metallicities. This challenges the current scenarios assuming that each feature in the disk (the 10 kpc ring) or in its outskirts (thick disk, the Giant Stream, and the numerous streams) is associated with an equivalent number of minor mergers. Given the large number of parameters, further constraints are certainly required to better render the complexity of M31 and of the substructures within its halo which may ultimately lead to a more precise geometry of the encounter. This would allow us, in principle, to evaluate the impact of such a major event on the Andromeda system and the Local Group.