Our Halo of Ice and Fire: Strong Kinematic Asymmetries in the Galactic Halo

TL;DR

This study reveals complex, asymmetric kinematic structures in the Galactic stellar halo, showing significant variations in velocity dispersion and orbit anisotropy, which challenge traditional symmetric models and impact mass distribution estimates.

Contribution

It provides the first detailed mapping of halo kinematics revealing strong asymmetries and heterogeneity, and distinguishes accreted from in-situ stars using chemistry, advancing understanding of Galactic assembly.

Findings

Halo exhibits azimuthal and mirror symmetry breaking.

Accreted stars have higher velocity dispersion and radial orbits.

In-situ stars show lower dispersion and isotropic orbits.

Abstract

The kinematics of the stellar halo hold important clues to the assembly history and mass distribution of the Galaxy. In this study, we map the kinematics of stars across the Galactic halo with the H3 Survey. We find a complex distribution that breaks both azimuthal symmetry about the $Z$-axis and mirror symmetry about the Galactic plane. This asymmetry manifests as large variations in the radial velocity dispersion $\sigma_r$ from as ``cold'' as 70 $\text{km}\text{ s}^{-1}$ to as ``hot'' as 160 $\text{km}\text{ s}^{-1}$. We use stellar chemistry to distinguish accreted stars from in-situ stars in the halo, and find that the accreted population has higher $\sigma_r$ and radially biased orbits, while the in-situ population has lower $\sigma_r$ and isotropic orbits. As a result, the Galactic halo kinematics are highly heterogeneous and poorly approximated as being spherical or axisymmetric. We measure radial profiles of $\sigma_r$ and the anisotropy parameter $\beta$ over Galactocentric radii $10-80\text{ kpc}$, and find that discrepancies in the literature are due to the nonspherical geometry and heterogeneous nature of the halo. Investigating the effect of strongly asymmetric $\sigma_r$ and $\beta$ on equilibrium models is a path forward to accurately constraining the Galactic gravitational field, including its total mass.