Chasing the formation history of the Galactic metal-poor disc

Abstract

In our previous work, we identified $\sim100,000$ metal-poor stars ([Fe/H] $<$ -1.0) from the LAMOST Survey. This work estimates their chemical abundances and explores the origin and evolution of the Galactic metal-poor disc. Our chemo-dynamical analysis reveals four main populations within the metal-poor disc: (1) a primordial disc older than 12 Gyr with [Fe/H] $>$ -1.5; (2) debris stars from the progenitor galaxy of Gaia-Sausage-Enceladus (GSE), but now residing in the Galactic disc; (3) the metal-poor tail of the metal-rich, high-$\alpha$ disc formed 10-12 Gyr ago, with metallicity lower limit extending to -2.0; (4) the metal-poor tail of the metal-rich, low-$\alpha$ disc younger than 8 Gyr, reaching a lower metallicity limit of -1.8. These results reveal the presence of a primordial disc and show that both high-$\alpha$ and low-$\alpha$ discs reach lower metallicities than previously thought. Analysis of merger debris reveals that Wukong, with extremely low metallicity, likely originate from merger events distinct from GSE. Additionally, three new substructures are identified: ShangGu-1, characterized by unusual [Fe/H]-eccentricity correlations; ShangGu-2, possibly heated disc stars; and ShangGu-3, which can be divided into four subgroups based on differing orbital directions, with two aligning with the previously known Nyx and Nyx-2.