Kavli Affiliate: Huawei Zhang
| Summary:
It has long been predicted that the Milky Way (MW) will eventually merge with Andromeda (M31), a view reinforced by textitHST measurements indicating a small M31 transverse velocity. However, using updated textitGaia-based proper motions (PMs) and including the dynamical influence of the Large Magellanic Cloud (LMC) and M33, Sawala et al. reported an MW–M31 merger probability of $sim$50% within 10 Gyr, leaving the fate of the Local Group (LG) uncertain. Adopting their semi-analytic framework, we revisit this problem with the latest and most precise textitGaia-based PMs for M31 and M33, corrected for systematic offsets in textitGaia astrometry. In our fiducial model, the MW–M31 merger probability rises to 90%, with a median merger time of $6.5_-1.5^+1.3$ Gyr, broadly restoring the classical picture. A sensitivity analysis shows that the merger probability depends strongly on the adopted M31 PM through two channels: a direct effect via the radial-tangential balance of the MW-M31 orbit, and a satellite-mediated effect, where the M31 PM fixes the orbital plane and determines how satellite-induced barycentric reflex motions project onto it, either promoting or suppressing a merger. Given this sensitivity, current measurements, while favoring a high merger probability, remain inconclusive, spanning from 64.7% to 100% across the 2$σ$ PM region. Future PM measurements with uncertainty of $lesssim2,upmumathrmas,yr^-1$ will be required to reach a firm conclusion, i.e., to constrain the probability range within 10% at the 2$σ$ level.
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