Revisiting the Role of M31 in the Dynamical History of the Magellanic Clouds

TL;DR

This study investigates the influence of M31 on the Magellanic Clouds' orbit within the Local Group, finding that M31's tidal effects are generally insignificant unless certain parameters are met, leaving the origin of their high angular momentum unresolved.

Contribution

The paper provides a comprehensive analysis of M31's role in the Magellanic Clouds' dynamics using a constrained Local Group model, clarifying when M31 can significantly affect their orbit.

Findings

M31's tides are mostly insignificant across parameter space.

M31 can influence the LMC's orbit if proper motions are high and total mass is large.

The combined MW-LMC mass is approximately 8.7 x 10^{11} solar masses.

Abstract

We study the dynamics of the Magellanic Clouds in a model for the Local Group whose mass is constrained using the timing argument/two-body limit of the action principle. The goal is to evaluate the role of M31 in generating the high angular momentum orbit of the Clouds, a puzzle that has only been exacerbated by the latest $HST$ proper motion measurements. We study the effects of varying the total Local Group mass, the relative mass of the Milky Way and M31, the proper motion of M31, and the proper motion of the LMC on this problem. Over a large part of this parameter-space we find that tides from M31 are insignificant. For a range of LMC proper motions approximately $3\sigma$ higher than the mean and total Local Group mass $> 3.5\times 10^{12} M_\odot$, M31 can provide a significant torque to the LMC orbit. However, if the LMC is bound to the MW, then M31 is found to have negligible effect on its motion and the origin of the high angular momentum of the system remains a puzzle. Finally, we use the timing argument to calculate the total mass of the MW-LMC system based on the assumption that they are encountering each other for the first time, their previous perigalacticon being a Hubble time ago, obtaining $M_{\rm MW} + M_{\rm LMC} = (8.7 \pm 0.8) \times 10^{11} M_\odot$.