# The effects of dynamical substructure on Milky Way mass estimates from the high velocity tail of the local stellar halo

## TL;DR

This study examines how dynamical substructures like streams affect estimates of the Milky Way's mass derived from local high velocity stars, revealing biases and proposing corrected mass values.

## Contribution

It demonstrates the significant impact of phase space substructure on mass estimates and provides a revised Milky Way mass accounting for these biases.

## Key findings

- Substructure causes variability in local velocity distributions.
- Mass estimates are biased low by about 20% without correction.
- Corrected Milky Way mass estimate is approximately 1.29 x 10^{12} solar masses.

## Abstract

We investigate the impact of dynamical streams and substructure on estimates of the local escape speed and total mass of Milky Way-mass galaxies from modelling the high velocity tail of local halo stars. We use a suite of high-resolution, magneto-hydrodynamical cosmological zoom-in simulations, which resolve phase space substructure in local volumes around solar-like positions. We show that phase space structure varies significantly between positions in individual galaxies and across the suite. Substructure populates the high velocity tail unevenly and leads to discrepancies in the mass estimates. We show that a combination of streams, sample noise and truncation of the high velocity tail below the escape speed leads to a distribution of mass estimates with a median that falls below the true value by $\sim 20 \%$, and a spread of a factor of 2 across the suite. Correcting for these biases, we derive a revised value for the Milky Way mass presented in Deason et al. of $1.29 ^{+0.37}_{-0.47} \times 10^{12}$ $\rm M_{\odot}$.