The shape of the inner Milky Way halo from observations of the Pal 5 and GD-1 stellar streams

TL;DR

This study uses stellar streams to measure the Milky Way's gravitational potential shape, finding a nearly spherical dark matter halo within 20 kpc, challenging simulation predictions.

Contribution

It provides new constraints on the Milky Way's halo shape and mass distribution using dynamical modeling of Pal 5 and GD-1 streams, with improved potential flattening measurements.

Findings

Halo potential flattening is approximately 0.95 at GD-1 and Pal 5 locations.

Dark matter halo's axis ratio is about 1.05 within 20 kpc.

Halo mass within 20 kpc is approximately 1.1 x 10^{11} solar masses.

Abstract

We constrain the shape of the Milky Way's halo by dynamical modeling of the observed phase-space tracks of the Pal 5 and GD-1 tidal streams. We find that the only information about the potential gleaned from the tracks of these streams are precise measurements of the shape of the gravitational potential---the ratio of vertical to radial acceleration---at the location of the streams, with weaker constraints on the radial and vertical accelerations separately. The latter will improve significantly with precise proper-motion measurements from Gaia. We measure that the overall potential flattening is 0.95 +/- 0.04 at the location of GD-1 ([R,z] ~ [12.5,6.7] kpc) and 0.94 +/- 0.05 at the position of Pal 5 ([R,z] ~ [8.4,16.8] kpc). Combined with constraints on the force field near the Galactic disk, we determine that the axis ratio of the dark-matter halo's density distribution is 1.05 +/- 0.14 within the inner 20 kpc, with a hint that the halo becomes more flattened near the edge of this volume. The halo mass within 20 kpc is 1.1 +/- 0.1 x 10^{11} M_sun. A dark-matter halo this close to spherical is in tension with the predictions from numerical simulations of the formation of dark-matter halos.