Partitioning the Galactic Halo with Gaussian Mixture Models

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Xilong Liang, Yuqin Chen, Jingkun Zhao, Gang Zhao

The Galactic halo is supposed to form from merging with nearby dwarf galaxies. In order to probe different components of the Galactic halo, we have applied the Gaussian Mixture Models method to a selected sample of metal poor stars with [Fe/H] <−0.7 dex in the APOGEE DR16 catalogue based on four-parameters, metallicity, [Mg/Fe] ratio and spatial velocity (\textit{VR, Vϕ}). Nine groups are identified with four from the halo (group 1, 3, 4 and 5), one from the thick disk (group 6), one from the thin disk (group 8) and one from dwarf galaxies (group 7) by analyzing their distributions in the ([M/H], [Mg/Fe]), (VR, Vϕ), (\textit{Zmax}, \textit{eccentricity}), (\textit{Energy}, \textit{Lz}) and ([Mg/Mn], [Al/Fe]) coordinates. The rest two groups are respectively caused by observational effect (group 9) and the cross section component (group 2) between the thin disk and the thick disk. It is found that in the extremely outer accreted halo (group 1), stars born in the Milky Way can not be distinguished from those accreted from other galaxies either chemically or kinematically. In the intermediate metallicity of −1.6 < [Fe/H] <−0.7 dex, the accreted halo mainly composed of the Gaia-Enceladus-Sausage substructure (group 5), which can be easily distinguished from group 4 (the in-situ halo group) in both chemical and kinematic space. Some stars of group 4 may come from the disk and some disk stars can be scattered to high orbits by resonant effects as shown in the \textit{Zmax} versus Energy coordinate. We also displayed the spatial distribution of main components of the halo and the ratio of accreted components do not show clear relation to Galactic radius.

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