Evidence for multiple crossings and stripping of Gaia-Enceladus/Sausage across the Milky Way

TL;DR

This study provides evidence that Gaia-Enceladus/Sausage was accreted onto the Milky Way in multiple passages, revealing distinct stellar populations with different chemical and dynamical properties, indicating a complex, multi-phase merger history.

Contribution

The paper demonstrates the existence of at least two separate stellar populations within Gaia-Enceladus, supporting a multi-passage accretion scenario supported by chemical evolution models.

Findings

Identification of two distinct populations with different chemical and dynamical properties.

Confirmation of a metallicity gradient within Gaia-Enceladus.

Support for a multi-passage accretion event through the Milky Way disc.

Abstract

The accretion of Gaia-Enceladus/Sausage (GES) onto the Milky Way (MW) is one of the most prominent features of the Galactic halo revealed by the combination of the Gaia satellite and large spectroscopic surveys. This massive accretion largely contributes to the local stellar halo mass and was significant enough to alter the formation history and the morphology of the MW. In this work, we aim to analyse the selection of stars previously identified as belonging to GES with different kinematics and chemical properties to test the hypothesis of a two-phase accretion event. We apply several statistical tests to assess the significance of the separation between the two populations in GES. We then employ galactic chemical evolution models to investigate the origin of the chemical differences encountered in the analysis. We confirm the presence of two distinct populations, with consistently different dynamical and chemical properties. The low energy population seems to show higher overall abundances, whereas the high-energy one may be more metal-poor. We attribute this difference to the presence of at least two separate populations of stars within Gaia-Enceladus, likely associated with the innermost (low-energy) and outermost (high-energy) regions of the progenitor. The adopted models successfully reproduce the patterns in metallicity and [alpha/M] distributions in an inside-out scenario. Our analysis supports the presence of a former metallicity gradient in Gaia-Enceladus, and reinforces the interpretation of its accretion as a multi-passage event through the Milky Way disc.