http://arxiv.org/abs/1802.03351
We analyse the structure of the local stellar halo of the Milky Way in action space using the SDSS–Gaia catalogue. 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 ($q\approx 0.9$). The highest energy stars are predominantly metal-rich and strongly retrograde, suggesting an origin in a retrograde encounter, possibly the one that created the stripped dwarf galaxy nucleus, $\omega$Centauri. We argue the metal-rich component is most likely built from recent accretion of a single massive dwarf galaxy. The metal-poor component may be the residue of material lost from globular clusters, or the early accretion of small dwarfs. Also visible as a distinct entity in action space is a component related to the thick disk, 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. This suggests the sculpting effect of a dynamical resonance, probably due to the Galactic bar and linked to the Hercules stream, is present at extremely low metallicities.
G. Myeong, N. Evans, V. Belokurov, et. al.
Mon, 12 Feb 18
2/53
Comments: ApJ (Letters), submitted