Does M31 result from an ancient major merger?
F. Hammer (1), Y. B. Yang (2), J. L. Wang (1,2), M. Puech (1), H., Flores (1), S. Fouquet (1) ((1) Laboratoire GEPI, Observatoire de Paris, (2) National Astronomical Observatories, Chinese Academy of Sciences (NAOC))

TL;DR
This study uses simulations to propose that M31's complex halo features and stellar populations can be explained by a single ancient major merger, challenging previous minor merger scenarios.
Contribution
It demonstrates that a single, ancient, gas-rich major merger can account for M31's halo structures, stellar populations, and morphological features, providing an alternative to multiple minor merger models.
Findings
A 3+/-0.5:1 gas-rich merger explains M31's properties.
Tidal tails and returning stars form the Giant Stream.
Predicted large loops match observed outskirts features.
Abstract
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. With an almost quiescent star formation history before the fusion we retrieve fractions of bulge, thin and thick disks as well as relative fractions of intermediate age and old stars in both the thick disk and the Giant Stream. The Giant Stream is caused by returning stars from a tidal tail previously stripped from the satellite prior to the fusion. These returning stars are trapped into elliptical orbits or loops for almost a Hubble time…
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