A simulation-based inference of the Milky Way merger history

Abstract

Accreted stars in the Milky Way (MW) preserve information about their galaxies of origin in their chemo-dynamical properties. In this study, we use the chemo-dynamical signatures in the merger debris to approximate the posterior distribution of the properties of disrupted satellites at their accretion. Adopting a simulation-based inference framework, we train an ensemble of normalizing flows using samples of merger debris from the Auriga simulations. Applying this methodology to a local sample of accreted stars in the MW, we infer the lookback times, stellar and halo masses, and halo mass merger ratios of several known accretion events: Gaia Enceladus-Sausage (GES), Helmi streams, Heracles, I’itoi, LMS-1/Wukong, Sagittarius, Sequoia and Thamnos. Our predictions for accretion time and mass align with previous estimates and follow the expected stellar mass - metallicity relation across redshifts. The total accreted stellar mass from these events is predicted to be $2.2^{+1.1}_{-0.6}\times 10^{9}~\rm {M_{\sun }}$, with the GES and Sagittarius dwarfs being the largest contributors. The total accreted stellar mass from all disrupted progenitors is $1.4^{+1.1}_{-0.4}\times 10^{9}~\rm {M_{\sun }}$, which is consistent with previous mass measurements of this component. We provide a prediction for the evolution of the MW halo mass until the accretion of Sagittarius (z ≈ 1): we find that the halo mass growth from the time of its first merger (z ≈ 5) to z ≈ 2 significantly exceeds the total mass of the known progenitors accreted during that interval, suggesting the presence of unidentified substructures. Our estimate of the Galaxy halo mass after the Sagittarius merger, but prior to the accretion of the Magellanic Clouds, is $5.9^{+1.4}_{-1.1}\times 10^{11}~\rm {M_{\sun }}$.