Low-alpha Metal-Rich Stars with Sausage Kinematics in the LAMOST Survey: Are they from the Gaia-Sausage-Enceladus Galaxy?

Abstract: We search for metal-rich Sausage-kinematic (MRSK) stars with [Fe/H]> -0.8 and -100<Vphi<50 km/s in LAMOST DR5 in order to investigate the influence of the Gaia-Sausage-Enceladus (GSE) merger event on the Galactic disk. For the first time, we find a group of low-alpha MRSK stars, and classify it as a metal-rich tail of the GSE galaxy based on the chemical and kinematical properties. This group has slightly larger Rapo, Zmax and Etot distributions than a previously-reported high-alpha group. Its low-alpha ratio does not allow for an origin resulting from the splash process of the GSE merger event, as is proposed to explain the high-alpha group. A hydrodynamical simulation by Amarante et al. provides a promising solution, in which the GSE galaxy is a clumpy Milky-Way analogue that develops a bimodal disk chemistry. This scenario explains the existence of MRSK stars with both high-alpha and low-alpha ratios found in this work. It is further supported by another new feature that a clump of MRSK stars is located at Zmax=3-5 kpc, which corresponds to the widely adopted disk-halo transition at |Z|~4 kpc. We suggest that a pile-up of MRSK stars at Zmax contributes significantly to this disk-halo transition, an interesting imprint left by the GSE merger event. These results also provide an important implication on the connection between the GSE and the Virgo Radial Merger.