Dynamical substructures of local metal-poor halo

Abstract

Based on 4098 very metal poor (VMP) stars with six-dimensional phase-space and chemical information from Gaia Data Release 3 (DR3) and Large sky Area Multi-Object fiber Spectroscopic Telescope DR9 as tracers, we apply an unsupervised machine learning algorithm, shared nearest neighbour, to identify stellar groups in the action-energy (J-E) space. We detect seven previously known mergers in local samples, including Helmi Stream, Gaia-Sausage/Enceladus (GSE), metal-weak thick disc (MWTD), Pontus, Wukong, Thamnos, and I'itoi + Sequoia + Arjuna. According to energy, we further divide GSE and Wukong into smaller parts to explore the orbital characteristics of individual fragments. Similarly, the division of Thamnos is based on action. It can be found that the apocentric distances of GSE parts of high and medium energy levels are located at and, respectively, which suggests that GSE could account for breaks in the density profile of the Galactic halo at both ≈30 and 15-18 kpc. The VMP stars of MWTD move along prograde orbits with larger eccentricities than those of its more metal-rich stars, which indicates that the VMP part of MWTD may be formed by accreting with dwarf galaxies. Finally, we summarize all substructures discovered in our local VMP samples. Our results provide a reference for the formation and evolution of the inner halo of the Milky Way.