The Milky Way Halo in Action Space

Abstract

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 phgr or J z . It has a mild prograde rotation $(\langle {v}{\phi }\rangle \approx 25,\mathrm{km},{{\rm{s}}}^{-1}$), 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 $(\langle {v}{\phi }\rangle \approx 50,\mathrm{km},{{\rm{s}}}^{-1}$), 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 $\langle {v}_{R}\rangle \approx 12,\mathrm{km},{{\rm{s}}}^{-1}$ within the solar circle at $| z| \lt 3.5,\mathrm{kpc}$. The existence of resonant stars at such extremely low metallicities has not been seen before.