Chemical Components in the Virgo Overdensity and Hercules-Aquila Cloud: hints of more than one merger event in GSE-like debris (2504.01478v1)

Abstract: Using elemental abundances for 1.26 million K giants in the LAMOST DR8 value-added catalog, we analyze the chemical abundances of the Virgo Overdensity (VOD) and Hercules-Aquila Cloud (HAC). We find two distinct chemical populations in both overdensities, which is in disagreement with the mainstream hypothesis that both overdensities are composed of materials from a single merger event, namely Gaia-Sausage-Enceladus (GSE). The two populations show different chemical trends: one exhibits low metallicities and high $\alpha$ abundances, and the other shows high metallicities and low $\alpha$ abundances, which is associated with the recently discovered Nereus and Virgo Radial Merger (VRM) components in the local stellar halo, respectively. The Nereus component in these overdensities uniquely exhibits a decreasing trend in the [Fe/H]-[Mn/Fe] plane. Out of all observed Milky Way dwarf galaxies, this trend is only found in the Sculptor dwarf galaxy, which provides clues for the properties of Nereus progenitor. We also find that the velocity ellipse with high aniostropy parameters that is usually considered to be part of GSE are actually a mix of the two components. Both overdensities are well-mixed in kinematic spaces, confirming recent claims that the debris of merger pairs are kinematically indistinguishable in a recent simulation. We find that the velocity ellipses of the VRM stars in these overdensities have large inclination angles, which may be an indication of the merger time in simulated merger events.