The orbital motions of halo stars in the Milky Way reflect the orbital motions of the progenitor systems in which they formed , making it possible to trace the mass-assembly history of the Galaxy .
Direct measurement of three-dimensional velocities , based on accurate proper motions and line-of-sight velocities , has revealed that the majority of halo stars in the inner-halo region move on eccentric orbits .
However , our understanding of the motions of distant , in-situ halo-star samples is still limited , due to the lack of accurate proper motions for these stars .
Here we explore a model-independent analysis of the line-of-sight velocities and spatial distribution of a recent sample of 1865 carefully selected halo blue horizontal-branch ( BHB ) stars within 30 { kpc } of the Galactic center .
We find that the mean rotational velocity of the very metal-poor ( { [ Fe / H ] } < -2.0 ) BHB stars significantly lags behind that of the relatively more metal-rich ( { [ Fe / H ] } > -2.0 ) BHB stars .
We also find that the relatively more metal-rich BHB stars are dominated by stars with eccentric orbits , as previously observed for other stellar samples in the inner-halo region .
By contrast , the very metal-poor BHB stars are dominated by stars on rounder , lower-eccentricity orbits .
Our results indicate that the motion of the progenitor systems of the Milky Way that contributed to the stellar populations found within 30 { kpc } correlates directly with their metal abundance , which may be related to their physical properties such as gas fractions .
These results are consistent with the existence of an inner/outer halo structure for the halo system , as advocated by Carollo et al .