Galactic googly: the rotation-metallicity bias in the inner stellar halo
  of the Milky Way
  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 ($v_{\rm rot}$) 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 $v_{\rm rot} \sim
20-40$ 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.
