The Milky Way Halo in Action Space

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 _ϕ or J _z . It has a mild prograde rotation (< {v}_φ > ≈ 25 {km} {{{s}}}^-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 > ≈ 50 {km} {{{s}}}^-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> ≈ 12 {km} {{{s}}}^-1 within the solar circle at | z| < 3.5 {kpc}. The existence of resonant stars at such extremely low metallicities has not been seen before.