Satellite galaxies as better tracers of the Milky Way halo mass

TL;DR

Satellite galaxies serve as more accurate tracers of the Milky Way's halo mass than halo stars, due to their closer phase space distribution alignment with dark matter particles, enabling more precise mass estimates with fewer tracers.

Contribution

This study demonstrates that satellite galaxies provide a more reliable and less biased method for inferring the Milky Way's halo mass compared to halo stars, due to their phase space properties.

Findings

Satellite galaxies closely trace dark matter phase space distribution.

Using ~100 satellite galaxies yields higher accuracy than larger halo star samples.

Phase correlations induce stochastic bias, larger for halo stars.

Abstract

The inference of the Milky Way halo mass requires modelling the phase space structure of dynamical tracers, with different tracers following different models and having different levels of sensitivity to the halo mass. For steady-state models, phase correlations among tracer particles lead to an irreducible stochastic bias. This bias is small for satellite galaxies and dark matter particles, but as large as a factor of 2 for halo stars. This is consistent with the picture that satellite galaxies closely trace the underlying phase space distribution of dark matter particles, while halo stars are less phase-mixed. As a result, the use of only $\sim 100$ satellite galaxies can achieve a significantly higher accuracy than that achievable with a much larger sample of halo stars.