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

This is the authors' abstract. We don't add key points for this paper.

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 σᵣ from as ``cold'' as 70 km s⁻¹ to as ``hot'' as 160 km 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 σᵣ and radially biased orbits, while the in-situ population has lower σᵣ 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 σᵣ and the anisotropy parameter β over Galactocentric radii 10-80 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 σᵣ and β on equilibrium models is a path forward to accurately constraining the Galactic gravitational field, including its total mass.