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 rpericenter=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 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 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 are belonging 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