The Milky Way Halo in Action Space

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Myeong, GC 
Evans, NW 
Sanders, JL 
Koposov, SE 

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 (⟨vϕ⟩≈25kms−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 (⟨vϕ⟩≈50kms−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 vR⟩≈12kms−1 within the solar circle at |z|<3.5kpc. The existence of resonant stars at such extremely low metallicities has not been seen before.

Galaxy: halo, Galaxy: stellar content, Galaxy: structure
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Astrophysical Journal Letters
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IoP Publishing
Science and Technology Facilities Council (ST/N000927/1)
European Research Council (308024)
G.C.M. thanks the Boustany Foundation, Cambridge Commonwealth, European & International Trust and Isaac Newton Studentship for their support of his work. J.L.S. thanks the Science and Technology Facilities Council for financial support. The research leading to these results has received partial support from the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013) / ERC Grant agreement No. 308024. This work has made use of data from the European Space Agency (ESA) mission Gaia (, processed by the Gaia Data Processing and Analysis Consortium (DPAC, Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement.