Evidence of a common origin for the Virgo overdensity and Hercules-Aquila Cloud from abundances and orbital parameters

Abstract

Aims. The Virgo overdensity (VOD) and the Hercules-Aquila cloud (HAC) may have originated from the same accretion event. In this work, we use K giants from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) survey to further investigate their potentially common origins. Methods. We selected member stars in the VOD and HAC regions from the K giant sample of LAMOST DR5 and cross-matched them with results from the literature to obtain their elemental abundances. The orbital characteristics, namely, eccentricity, apocenter distance, energy, and angular momentum were compared between the member stars in the VOD and HAC regions. Then, we investigated the relationship between the VOD and HAC from the perspective of chemical evolution through a comparison of the distributions of elemental abundances in these two regions. Results. By studying the orbital parameters of the members in VOD and HAC, we find that the distribution of the orbital eccentricity, apocenter, and maximum height from the Galactic disk of the members of these two regions are very consistent. They are both on orbits of low angular momentum and high energy. Their density distributions in the spatial position are also similar based on an integration of the last 8 Gyr of their orbits. We also found that these two structures have similar distributions in [Fe/H] and other elemental abundances. Based on the similarity of the orbital properties and the consistency of the chemical abundances, we suggest that these two structures may have come from the merger of the same dwarf galaxy, such as the Gaia-Sausage-Enceladus (GSE). Also, part of the VOD may originate from Small Magellanic Cloud (SMC) debris that had been stripped 3 Gyr ago.