The Masses of the Milky Way and Andromeda galaxies

TL;DR

This paper introduces robust tracer mass estimators to determine galaxy halo masses from discrete kinematic data, applying them to the Milky Way and Andromeda, and quantifying uncertainties through simulations.

Contribution

The paper develops new mass estimators tailored for galaxy satellites and applies them to Milky Way and M31 data, providing updated mass estimates with uncertainty analysis.

Findings

Milky Way mass within 300 kpc is ~0.9 x 10^12 solar masses assuming isotropy.

Including proper motions increases the Milky Way mass estimate to ~1.4 x 10^12 solar masses.

M31 mass within 300 kpc is estimated at ~1.4 x 10^12 solar masses with modest dependence on anisotropy.

Abstract

We present a family of robust tracer mass estimators to compute the enclosed mass of galaxy haloes from samples of discrete positional and kinematical data of tracers, such as halo stars, globular clusters and dwarf satellites. The data may be projected positions, distances, line of sight velocities or proper motions. Forms of the estimator tailored for the Milky Way galaxy and for M31 are given. Monte Carlo simulations are used to quantify the uncertainty as a function of sample size. For the Milky Way, the satellite sample consists of 26 galaxies with line-of-sight velocities. We find that the mass of the Milky Way within 300 kpc is ~ 0.9 x 10^12 solar masses assuming velocity isotropy. However, the mass estimate is sensitive to the anisotropy and could plausibly lie between 0.7 - 3.4 x 10^12 solar masses. Incorporating the proper motions of 6 Milky Way satellites into the dataset, we find ~ 1.4 x 10^12 solar masses. The range here if plausible anisotropies are used is still broader, from 1.2 - 2.7 x 10^12 solar masses. For M31, there are 23 satellite galaxies with measured line-of-sight velocities, but only M33 and IC 10 have proper motions. We use the line of sight velocities and distances of the satellite galaxies to estimate the mass of M31 within 300 kpc as ~ 1.4 x 10^12 solar masses assuming isotropy. There is only a modest dependence on anisotropy, with the mass varying between 1.3 -1.6 x 10^12 solar masses. Given the uncertainties, we conclude that the satellite data by themselves yield no reliable insights into which of the two galaxies is actually the more massive.