The Shape and Mass of the Galactic Dark Matter Halo from the Axisymmetric Jeans Model

Abstract

We explore the density profile, shape, and virial mass of the Milky Way’s dark matter halo, using K giants (KGs) from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope and Sloan Digital Sky Survey (SDSS)/SEGUE as well as blue horizontal branch stars (BHBs) from SDSS. Incorporating Gaia DR3 proper motions, we first investigate the velocity ellipsoid distribution within the ( R , ∣ z ∣) space. The ellipsoids projected onto the ( v _R , v _z ) plane exhibit near-spherical alignment. We then probe the underlying dark matter distribution, using the axisymmetric Jeans equations with multi-Gaussian expansion and spherically aligned Jeans anisotropic modeling (JAMsph), allowing for different flattened dark matter density models. For each model, we apply two fitting approaches: fitting the KGs and BHBs separately or fitting them simultaneously as two dynamical tracers in one gravitational potential. We find consistent results for the dark matter density profiles, r _200 , and M _200 within a 1 σ confidence region for models constrained by KGs, BHBs, and both. We find the strongest consistency between KGs and BHBs in constraining dark matter profiles for models incorporating radially varying halo flattening ( q ( r _gc )), which suggests the Milky Way’s dark matter halo shape evolves with Galactocentric distance ( r _gc ). Specifically, the halo flattening parameter q _h decreases within r _gc 20 kpc. In this model, ${M}_{\mathrm{tot}}(\lt 60\,\mathrm{kpc})=0.53{3}_{-0.054}^{+0.061}\times 1{0}^{12}$ M _⊙ , r _200 is 188 ± 15 kpc, and M _200 is estimated at $0.82{0}_{-0.186}^{+0.210}\times 1{0}^{12}{M}_{\odot }$ .

Keywords

• Horizontal branch stars
• K giant stars
• Milky Way mass
• Milky Way dark matter halo