2025ApJ...987..143Z

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, r₂₀₀, and M₂₀₀ within a 1σ 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 and increases for 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_⊙, r₂₀₀ is 188 ± 15 kpc, and M₂₀₀ is estimated at $0.82{0}_{-0.186}^{+0.210}\times 1{0}^{12}{M}_{\odot }$.