Estimating Initial Mass of Gaia-Enceladus Dwarf Galaxy with Chemical Evolution Model

Abstract: This work investigates the initial mass and chemical evolution history of the Gaia-Enceladus dwarf galaxy. We combine spectroscopic data from APOGEE with astrometric data from Gaia DR3 to identify Gaia-Enceladus candidate stars via a machine-learning pipeline using t-SNE and HDBSCAN. By focusing on kinematic and chemical parameters, especially $\mathrm{[Fe/H]}$, $\mathrm{[Mg/Fe]}$, $\mathrm{[Al/Fe]}$, and $\mathrm{[Mn/Fe]}$, we uncover a population of metal-poor, high-eccentricity stars that align with literature criteria for Gaia-Enceladus debris. We then apply the \textit{OMEGA+} chemical evolution model, incorporating MCMC fitting of the observed abundance trends in the $\mathrm{[Mg/Fe]\times[Fe/H]}$ plane. Our best-fitting model indicates a gas mass of $4.93_{-0.72}^{+0.32}\times10^9\,{M_{\odot}}$ for Gaia-Enceladus, placing it at the higher end of previously suggested mass ranges. The model scenario suggests a short star formation timescale, substantial outflows, and a rapid build-up of metals mainly driven by core-collapse supernovae, with a lesser contribution from Type~Ia supernovae. Comparison with observational data in other chemical planes (e.g., $\mathrm{[Mg/Mn]\times[Al/Fe]}$) supports this scenario, emphasizing a distinct evolution path relative to the Milky Way. Additionally, our results provide indirect evidence that star formation in Gaia-Enceladus likely ceased within the first 4 Gyr, consistent with earlier inferences of an early merger event. These findings highlight the power of chemical evolution modeling in reconstructing the origin and mass of ancient accreted systems. Overall, we show that Gaia-Enceladus, through a rapid star formation and strong outflows, contributed a significant fraction of the metal-poor stellar halo of the Milky Way.