On the Adromeda to Milky Way mass-ratio

TL;DR

This study estimates the mass-ratio of Andromeda to the Milky Way by analyzing tidal forces and the zero-tidal shell, suggesting M31 is significantly more massive with a ratio between 2 and 3.

Contribution

It introduces a method to constrain galaxy mass-ratios using tidal radius measurements and the zero-tidal shell concept in the Local Group.

Findings

Mass-ratio between 2 and 3 is favored.

M31 is significantly more massive than the Milky Way.

Tidal forces can effectively constrain galaxy mass ratios.

Abstract

We have explored the hypothesis that the total mass-ratio of the two main galaxies of the Local Group: Andromeda Galaxy(M31) and the Milky Way (MW) can be constrained measuring the tidal force induced by the surrounding mass distribution, M31 included, on the MW. We argue that the total mass-ratio between the two groups can be approximated, at least qualitatively, finding the tidal radius where the internal binding force of the MW balances the external tidal force acting on it. Since M31 is the massive tidal perturber of the local environment, we have used a wide range of M31 to MW mass-ratio combinations to compute the corresponding tidal radii. Of them, only few match the distance of the zero-tidal shell i.e. the shell identified observationally by the outermost dwarf galaxies which do not show any sign of tidal effects. This is the key to constrain the best mass-ratio interval of the two galaxies. Our results favour a solution where the mass-ratio ranges from 2 to 3 implying a massive predominance of M31.