The Space Motion of Leo I: The Mass of the Milky Way's Dark Matter Halo

TL;DR

This study combines Hubble measurements and high-resolution simulations to constrain the Milky Way's dark matter halo mass using Leo I's motion, finding it likely exceeds 10^{12} solar masses and emphasizing the importance of proper motions in satellite history analysis.

Contribution

It provides new constraints on the Milky Way's dark matter halo mass by integrating observational data with advanced simulations, highlighting the role of satellite proper motions.

Findings

Leo I's velocity suggests a Milky Way mass > 10^{12} solar masses.

Most simulated subhalos at Leo I's position are bound, supporting a massive halo.

Proper motions help interpret satellite infall times and orbital histories.

Abstract

We combine our Hubble Space Telescope measurement of the proper motion of the Leo I dwarf spheroidal galaxy (presented in a companion paper) with the highest resolution numerical simulations of Galaxy-size dark matter halos in existence to constrain the mass of the Milky Way's dark matter halo (M_MW). Despite Leo I's large Galacto-centric space velocity (200 km/s) and distance (261 kpc), we show that it is extremely unlikely to be unbound if Galactic satellites are associated with dark matter substructure, as 99.9% of subhalos in the simulations are bound to their host. The observed position and velocity of Leo I strongly disfavor a low mass Milky Way: if we assume that Leo I is the least bound of the Milky Way's classical satellites, then we find that M_MW > 10^{12} M_sun at 95% confidence for a variety of Bayesian priors on M_MW. In lower mass halos, it is vanishingly rare to find subhalos at 261 kpc moving as fast as Leo I. Should an additional classical satellite be found to be less bound than Leo I, this lower limit on M_MW would increase by 30%. Imposing a mass weighted LCDM prior, we find a median Milky Way virial mass of M_MW=1.6 x 10^{12} M_sun, with a 90% confidence interval of [1.0-2.4] x 10^{12} M_sun. We also confirm a strong correlation between subhalo infall time and orbital energy in the simulations and show that proper motions can aid significantly in interpreting the infall times and orbital histories of satellites.