2018ApJ...856L..26M

We analyze the structure of the local stellar halo of the Milky Way using ∼60000 stars with full phase space coordinates extracted from the SDSS-Gaia catalog. We display stars in action space as a function of metallicity in a realistic axisymmetric potential for the Milky Way Galaxy. The metal-rich population is more distended toward high radial action J_R as compared to azimuthal or vertical action, J_ph or J_z. It has a mild prograde rotation (<v_φ>~25kms^–1), is radially anisotropic and highly flattened, with axis ratio q ≃ 0.6-0.7. The metal-poor population is more evenly distributed in all three actions. It has larger prograde rotation (<v_φ>~50kms^–1), a mild radial anisotropy, and a roundish morphology (q ≃ 0.9). We identify two further components of the halo in action space. There is a high-energy, retrograde component that is only present in the metal-rich stars. This is suggestive of an origin in a retrograde encounter, possibly the one that created the stripped dwarf galaxy nucleus, ωCentauri. Also visible as a distinct entity in action space is a resonant component, which is flattened and prograde. It extends over a range of metallicities down to [Fe/H] ≃ -3. It has a net outward radial velocity <v_R>~12kms^–1 within the solar circle at | z| < 3.5kpc. The existence of resonant stars at such extremely low metallicities has not been seen before.