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We analyze the anisotropy profile of the Milky Way's smooth, diffuse stellarhalo using SDSS/SEGUE blue horizontal branch stars and SDSS/SEGUE and LAMOST Kgiants. These intrinsically luminous stars allow us to probe the halo toapproximately 100 kpc from the Galactic center. Line-of-sight velocities,distances, metallicities, and proper motions are available for all stars viaSDSS/SEGUE, LAMOST, and Gaia, and we use these data to construct a full 7D setconsisting of positions, space motions, and metallicity. We remove substructurefrom our samples using integrals of motion based on the method of Xue et al. Wefind radially dominated kinematic profiles with nearly constant anisotropywithin 20 kpc, beyond which the anisotropy profile gently declines althoughremains radially dominated to the furthest extents of our sample. Independentof star type or substructure removal, the anisotropy depends on metallicity,such that the orbits of the stars become less radial with decreasingmetallicity. For $-1.7<$ [Fe/H] $<-1$, the smooth, diffuse halo anisotropyprofile begins to decline at Galactocentric distances $\sim20$ kpc, from$\beta\sim0.9$ to 0.7 for K giants and from $\beta\sim0.8$ to 0.1 for bluehorizontal branch stars. For [Fe/H] $<-1.7$, the smooth, diffuse haloanisotropy remains constant along all distances with $0.2<\beta<0.7$ dependingon the metallicity range probed, although independent on star type. Thesesamples are ideal for estimating the total Galactic mass as they represent thevirialized stellar halo system.

astrophysical journal/the astrophysical journal

Constraints on the assembly history of the Milky Way's smooth, diffuse  stellar halo from the metallicity-dependent, radially-dominated velocity  anisotropy profiles probed with K giants and BHB stars using LAMOST,  SDSS/SEGUE, and Gaia