Unveiling Galactic substructures with M Giant stars: A kinematic and chemical study based on LAMOST DR9, Gaia DR3 and APOGEE DR17

Abstract: Based on the updated M giant star catalog selected from LAMOST DR9, we iden-tify substructures within the integrals-of-motion space through Friends-of-Friends cluster-ing algorithm. We obtain members belonging to several known substructures: the Sagittarius stream, Galactic Anticenter Substructure (GASS), Gaia-Enceladus-Sausage (GES), Splash, and the high-{\alpha} disk. Furthermore, we also identify two groups which cannot be clearly asso-ciated with previously known substructures. Our findings confirm the existence of metal-rich constituents within the GES, representing newly formed stars that originated from the metal-enriched gas delivered during the GES merger event and subsequently evolved. Additionally, this study further expands the sample of GASS, high-{\alpha} disk, and Splash stars. Analysis of these metal-rich M giant stars as members of the GES, Splash, and high-{\alpha} disk compo-nents supports an evolution scenario for the early Milky Way, as proposed by previous stud-ies. In this scenario, stars initially formed in a high-{\alpha} primordial disk were dynamically heated by the massive accretion event (GES). This process redistributed stellar orbits, creat-ing the Splash population, while the undisturbed portion of the primordial disk persisted as the present-day high-{\alpha} disk component.