Visit Nearby Halo Substructures Using LAMOST DR9 MRS Data

Abstract Based on the LAMOST DR9 Medium Resolution Spectra (MRS) catalog, combined with astrometries from Gaia DR3, we conducted an investigation into the substructures in the stellar halo to obtain their chemical and kinematic information. We employed the HDBSCAN algorithm in the ( L z , E , L xy ) space for clustering and utilized Mahalanobis distance to merge the clustering results. Ultimately, within the LAMOST DR9 MRS catalog, we identified members of Gaia–Sausage–Enceladus (GSE), Helmi streams, and the high- α disk and in situ halo. Additionally, there are three clusters that cannot be correlated with known substructures. Based on their angular momentum ( L z ) characteristics, we have tentatively named two of them High- L z -Cluster-1 (HLC-1) and High- L z -Cluster-2 (HLC-2), both exhibiting high angular momentum ( L z ) features. The third cluster, owing to its extremely low L z , is termed the Low- L z -Cluster. Then detailed analysis about those substructures are executed including member stars’ identification and kinematic and chemical distribution. The resulting GSE member stars exhibit a well-defined metallicity distribution function with its peak at Fe/H ∼ −1.2, consistent with previous findings, and we found there appear to be some differences in the distribution on the V total – r gal plane between GSE and its included Splash component. Regarding Helmi streams, in the Mg/Fe–Fe/H plane we detected potential traces of the α -knee phenomenon at Fe/H ∼ −1.53. Finally, we tentatively provide the possible origin for the HLC-1 and HLC-2.