The Fall of a Giant. Chemical evolution of Enceladus, alias the Gaia Sausage

In this Letter, we present the first chemical evolution model for Enceladus, alias the Gaia Sausage, to investigate the star formation history of one of the most massive satellites accreted by the Milky Way during a major merger event. Our best chemical evolution model for Enceladus nicely fits the observed stellar [$\alpha$/Fe]-[Fe/H] chemical abundance trends, and reproduces the observed stellar metallicity distribution function, by assuming low star formation efficiency, fast gas accretion time scale, and mild outflow intensity. We find that $80$ per cent of Enceladus stars have ages $>11.7$ Gyr. At the time of the merger with the Milky Way, we find for Enceladus a stellar mass $M_{\star} \approx 5 \times 10^{9} \text{M}_{\odot}$, with a gas fraction of $\sim 0.25$, if the assumed infall mass is $M_{\text{inf}}=10^{10}\,\text{M}_{\odot}$. The comparison between the age distribution of the stars in Enceladus from our best chemical evolution model and the predicted star formation history of the two-infall models of Noguchi (2018) and Spitoni et al. (2018) seems to suggest that a merger event between the Galaxy and systems like Enceladus could have been capable of inhibiting the gas accretion onto the Galaxy disc, heating up the gas in the halo at high redshifts. To firmly confirm or reject this hypothesis, however, we will need in the future more precise age measurements of Enceladus stars than those coming from isochrone-fitting analysis or chemical tagging like in this work, as achievable by means of asteroseismic techniques.