The Stellar Halo of the Galaxy is Tilted & Doubly Broken

TL;DR

This paper models the shape and density profile of the Milky Way's stellar halo, revealing it is tilted, triaxial, and features a double break in density, providing new insights into the galaxy's merger history and dark matter distribution.

Contribution

It introduces a detailed triaxial, tilted model of the stellar halo with a doubly-broken power law density profile, resolving previous discrepancies in halo break radius measurements.

Findings

The halo is a tilted, triaxial ellipsoid with axes ratios 10:8:7.

The density profile has two breaks at 12 kpc and 28 kpc.

The double break relates to orbital apocenters from the GSE merger.

Abstract

Modern Galactic surveys have revealed an ancient merger that dominates the stellar halo of our Galaxy (\textit{Gaia}-Sausage-Enceladus, GSE). Using chemical abundances and kinematics from the H3 Survey, we identify 5559 halo stars from this merger in the radial range $r_{\text{Gal}}=6-60\text{ kpc}$. We forward model the full selection function of H3 to infer the density profile of this accreted component of the stellar halo. We consider a general ellipsoid with principal axes allowed to rotate with respect to the Galactocentric axes, coupled with a multiply-broken power law. The best-fit model is a triaxial ellipsoid (axes ratios 10:8:7) tilted $25^\circ$ above the Galactic plane towards the Sun and a doubly-broken power law with breaking radii at 12 kpc and 28 kpc. This result resolves the long-standing dichotomy in literature values of the halo breaking radius, being at either $\sim15\text{ kpc}$ or $\sim30\text{ kpc}$ assuming a singly-broken power law. N-body simulations suggest that the breaking radii are connected to apocenter pile-ups of stellar orbits, and so the observed double-break provides new insight into the initial conditions and evolution of the GSE merger. Furthermore, the tilt and triaxiality of the stellar halo could imply that a fraction of the underlying dark matter halo is also tilted and triaxial. This has important implications for dynamical mass modeling of the Galaxy as well as direct dark matter detection experiments.