The Milky Way Halo in Action Space

G. C. Myeong; N. W. Evans; V. Belokurov; J. L. Sanders; S. Koposov

doi:10.3847/2041-8213/aab613

paper

The Milky Way Halo in Action Space

arXiv:1802.03351 · doi:10.3847/2041-8213/aab613

Abstract

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, $J_{ϕ}$ or $J_{z}$ . It has a mild prograde rotation $(⟨ v_{ϕ} ⟩ \approx 25$ 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 $(⟨ v_{ϕ} ⟩ \approx 50$ 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, $ω$ 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 $⟨ v_{R} ⟩ \approx 12$ 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.

ApJ (Letters), published

The Milky Way Halo in Action Space

Abstract

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