The Milky Way and M31 Orbital History: Did the Local Group evolve in isolation?

TL;DR

This study investigates the orbital history of the Milky Way and Andromeda, assessing whether the Local Group evolved in isolation by analyzing proper motion data, simulation comparisons, and the impact of mergers on their orbital dynamics.

Contribution

It provides new insights into the MW-M31 orbit using recent measurements and compares isolated two-body models with cosmological simulations to evaluate their accuracy and implications.

Findings

Approximately 13% of LG-like pairs had a pericentric passage.

About 10% of analogues are well-modeled by isolated two-body orbits.

Systems evolving in isolation have fewer major mergers since z ≈ 0.3.

Abstract

We use new measurements of the M31 proper motion to examine the Milky Way (MW) - M31 orbit and angular momentum. For Local Group (LG) mass consistent with measured values, and assuming the system evolves in isolation, we show a wide range of orbits is possible. We compare to a sample of LG-like systems in the Illustris simulation and find that $\sim 13\%$ of these pairs have undergone a pericentric passage. Using the simulated sample, we examine how accurately an isolated, two-body model describes the MW-M31 orbit, and show that $\sim 10\%$ of the analogues in the simulation are well-modeled by such an orbit. Systems that evolve in isolation by this definition are found to have a lower rate of major mergers and, in particular, have no major mergers since $z \approx 0.3$. For all systems, we find an increase in the orbital angular momentum, which is fairly independent of the merger rate and is possibly explained by the influence of tidal torques on the LG. Given the likely quiet recent major merger history of the MW, it is plausible that the isolated two-body model appropriately describes the orbit, though recent evidence for a major merger in M31 may complicate this interpretation.