DOES M31 RESULT FROM AN ANCIENT MAJOR MERGER?

Abstract

The numerous streams in the M31 halo are currently assumed to be due to\nmultiple minor mergers. Here we use the GADGET2 simulation code to test whether\nM31 could have experienced a major merger in its past history. It results that\na 3+/-0.5:1 gaseous rich merger with r(per)=25+/-5 kpc and a polar orbit can\nexplain many properties of M31 and of its halo. The interaction and the fusion\nmay have begun 8.75+/-0.35 Gyr and 5.5 +/-0.5 Gyr ago, respectively. With an\nalmost quiescent star formation history before the fusion we retrieve fractions\nof bulge, thin and thick disks as well as relative fractions of intermediate\nage and old stars in both the thick disk and the Giant Stream. The Giant Stream\nis caused by returning stars from a tidal tail previously stripped from the\nsatellite prior to the fusion. These returning stars are trapped into\nelliptical orbits or loops for almost a Hubble time period. Large loops are\nalso predicted and they scale rather well with the recently discovered features\nin the M31 outskirts. We demonstrate that a single merger could explain\nfirst-order (intensity and size), morphological and kinematical properties of\nthe disk, thick disk, bulge and streams in the halo of M31, as well as the\ndistribution of stellar ages, and perhaps metallicities. It challenges\nscenarios assuming one minor merger per feature in the disk (10 kpc ring) or at\nthe outskirts (numerous streams & thick disk). Further constraints will help to\nproperly evaluate the impact of such a major event to the Local Group.\n