The first and second moments of stellar velocities encode important information about the formation history of the Galactic halo .
However , due to the lack of tangential motion and inaccurate distances of the halo stars , the velocity moments in the Galactic halo have largely remained ‘ ‘ known unknowns ’ ’ .
Fortunately , our off-centric position within the Galaxy allows us to estimate these moments in the galacto-centric frame using the observed radial velocities of the stars alone .
We use these velocities coupled with the Hierarchical Bayesian scheme , which allows easy marginalisation over the missing data ( the proper-motion , and uncertainty-free distance and line-of-sight velocity ) , to measure the velocity dispersions , orbital anisotropy ( \beta ) and streaming motion ( v _ { rot } ) of the halo main-sequence turn-off ( MSTO ) and K-giant ( KG ) stars in the inner stellar halo ( r \lesssim 15 ~ { } { kpc } ) .
We study the metallicity bias in kinematics of the halo stars and observe that the comparatively metal-rich ( [ Fe/H ] > -1.4 ) and the metal-poor ( [ Fe/H ] \leq - 1.4 ) MSTO samples show a clear systematic difference in v _ { rot } \sim 20 - 40 km s ^ { -1 } , depending on how restrictive the spatial cuts to cull the disk contamination are .
The bias is also detected in KG samples but with less certainty .
Both MSTO and KG populations suggest that the inner stellar halo of the Galaxy is radially biased i.e . \sigma _ { r } > \sigma _ { \theta } or \sigma _ { \phi } and \beta \simeq 0.5 .
The apparent metallicity contrariety in the rotation velocity among the halo sub-populations supports the co-existence of multiple populations in the galactic halo that may have formed through distinct formation scenarios , i.e .
in-situ versus accretion .