Evidence of Gaia Enceladus experiencing at least two passages around the Milky Way

Abstract: One of the major recent breakthroughs has been the discovery of the last Major Merger to happen in the history of the Milky Way. Around 10 Gyr ago the galaxy Gaia Enceladus, with estimated ~10% of the Milky Way mass, fell into its potential, bringing a large amount of stars which can be identified through their unique chemical and kinematic signatures. Simulations have long predicted that a galaxy of this size should experience several passages through the disk of the Milky Way before eventually being fully dispersed. For the first time, we present observational evidence to support this. We identify two subpopulations accreted from Gaia Enceladus: 1) stars which today have large kinematic energy, which originate from the outskirts of Gaia Enceladus and were accreted during early passages; 2) stars with low kinetic energy accreted at later passages, originating from the inner parts of Gaia Enceladus. Through the use of high-precision chemical abundances, crucially including new aluminum measurements, we show that in all observed abundance ratios ([Fe/H], [Al/Fe], [Mg/Fe] and [Mg/Ba]), stars with high energy show evidence of coming from a less chemically evolved outer region of Gaia Enceladus, compared to the stars with low energy. We therefore conclude that Gaia Enceladus experienced several passages before merging with the main body of our Galaxy. This discovery has wide implications for our understanding of this event, and consolidates Gaia Enceladus as a benchmark for studying galaxy mergers and hierarchical galaxy formation in extraordinary details.