Does M31 result from an ancient major merger?

Understand

The numerous streams in the M31 halo are currently assumed to be due to multiple minor mergers.

• Here we use the GADGET2 simulation code to test whether M31 could have experienced a major merger in its past history.
• It results that a 3+/-0.5:1 gaseous rich merger with r(per)=25+/-5 kpc and a polar orbit can explain many properties of M31 and of its halo.
• The interaction and the fusion may have begun 8.75+/-0.35 Gyr and 5.5 +/-0.5 Gyr ago, respectively.