The Milky Way Halo in Action Space

We analyse the structure of the local stellar halo of the Milky Way using $\sim$ 60000 stars with full phase space coordinates extracted from the SDSS--{\it Gaia} catalogue. 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 towards high radial action $J_R$ as compared to azimuthal or vertical action, $J_\phi$ or $J_z$. It has a mild prograde rotation $(\langle v_\phi \rangle \approx 25$ km s$^{-1}$), is radially anisotropic and highly flattened with axis ratio $q \approx 0.6 - 0.7$. The metal-poor population is more evenly distributed in all three actions. It has larger prograde rotation $(\langle v_\phi \rangle \approx 50$ km s$^{-1}$), a mild radial anisotropy and a roundish morphology ($q\approx 0.9$). 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] $\approx -3$. It has a net outward radial velocity $\langle v_R \rangle \approx 12$ 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.