The Fate of the Milky Way--Andromeda System: To Merge or Not?

TL;DR

This study revisits the likelihood of the Milky Way and Andromeda merging within 10 billion years using updated Gaia data, revealing a high probability but emphasizing the need for more precise measurements to confirm the outcome.

Contribution

It provides a refined analysis of the MW--M31 merger probability incorporating latest Gaia proper motions and the influence of satellite galaxies, highlighting the sensitivity to measurement uncertainties.

Findings

Merger probability increases to 90% with updated data.

Merger time median is approximately 6.5 Gyr.

Current measurements are inconclusive, requiring more precise data.

Abstract

It has long been predicted that the Milky Way (MW) will eventually merge with Andromeda (M31), a view reinforced by \textit{HST} measurements indicating a small M31 transverse velocity. However, using updated \textit{Gaia}-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 \textit{Gaia}-based PMs for M31 and M33, corrected for systematic offsets in \textit{Gaia} 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$\sigma$ PM region. Future PM measurements with uncertainty of $\lesssim2\,\upmu\mathrm{as\,yr^{-1}}$ will be required to reach a firm conclusion, i.e., to constrain the probability range within 10\% at the 2$\sigma$ level.