Partitioning the Galactic halo with Gaussian Mixture Models

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...

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Bibliographic Details
Published in:Research in astronomy and astrophysics 2021-06, Vol.21 (5), p.128-282
Main Authors: Liang, Xi-Long, Chen, Yu-Qin, Zhao, Jing-Kun, Zhao, Gang
Format: Article
Language:English
Subjects:
Aluminum
Astronomical models
Dwarf galaxies
Enceladus
Galactic halos
Galaxy: abundances
Galaxy: kinematics
Galaxy: structure
Iron
Magnesium
Manganese
Metallicity
Milky Way
Probabilistic models
Spatial distribution
Stars
Stars & galaxies
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Summary: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 ( V R , V ϕ ). 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]), ( V R , V ϕ ), ( Zmax , eccentricity ), ( Energy , Lz ) and ([Mg/Mn], [Al/Fe]) coordinates. The rest of the 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 cannot 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 is 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 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 the Galactic radius.
ISSN:1674-4527
DOI:10.1088/1674-4527/21/5/128