The anisotropy parameter \beta characterizes the extent to which orbits in stellar systems are predominantly radial or tangential , and is likely to constrain , for the stellar halo of the Milky Way , scenarios for its formation and evolution .
We have measured the anisotropy \beta as a function of Galactocentric radius from 5 - 100 kpc for over 8600 metal poor ( [ Fe/H ] < -1.3 ) halo K giants from the LAMOST catalog with line-of-sight velocities and distances , matched to proper motions from the second Gaia data release .
We construct full 6-D positions and velocities for the K giants to directly measure the 3 components of the velocity dispersion ( \sigma _ { r } , \sigma _ { \theta } , \sigma _ { \phi } ) ( in spherical coordinates ) .
We find that the orbits in the halo are radial over our full Galactocentric distance range reaching over 100 kpc .
The anisotropy remains remarkably unchanged with Galactocentric radius from approximately 5 to 25 kpc , with an amplitude that depends on the metallicity of the stars , dropping from \beta \approx 0.9 for -1.8 \leq [ Fe/H ] < -1.3 ( for the bulk of the stars ) to \beta \approx 0.6 for the lowest metallicities ( [ Fe/H ] < -1.8 ) .
Considering our sample as a whole , \beta \approx 0.8 and , beyond 25 kpc , the orbits gradually become less radial and anisotropy decreases to \beta < 0.3 past 100 kpc .
Within 8 kpc , \beta < 0.8 .
The measurement of anisotropy is affected by substructure and streams , particularly beyond a Galactocentric distance of approximately 25 kpc , where the Sagittarius stream is prominent in the data .
These results are complimentary to recent analysis of simulations by Loebman et al .
and of SDSS/ Gaia DR1 data by Belokurov et al .