The Milky Way Halo in Action Space

We analyze the structure of the local stellar halo of the Milky Way using similar to 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(phi) or J(z). It has a mild prograde rotation (< v(phi)> approximate to 25km s(-1)), is radially anisotropic and highly flattened, with axis ratio q approximate to 0.6-0.7. The metal-poor population is more evenly distributed in all three actions. It has larger prograde rotation (< v(phi)> approximate to 50 km s(-1)), a mild radial anisotropy, and a roundish morphology (q approximate to 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, omega Centauri. 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] approximate to -3. It has a net outward radial velocity < v(R)> approximate to 12 km s(-1)), within the solar circle at vertical bar z vertical bar < 3.5 kpc. The existence of resonant stars at such extremely low metallicities has not been seen before.