Reconstructing the Last Major Merger of the Milky Way with the H3 Survey

Several lines of evidence suggest that the Milky Way underwent a major merger at z similar to 2 with the Gaia-Sausage-Enceladus (GSE) galaxy. Here we use H3 Survey data to argue that GSE entered the Galaxy on a retrograde orbit based on a population of highly retrograde stars with chemistry similar to the largely radial GSE debris. We present the first tailored N-body simulations of the merger. From a grid of approximate to 500 simulations we find that a GSE with M-* = 5 x 10(8) M-circle dot, M-DM = 2 x 10(11) M-circle dot best matches the H3 data. This simulation shows that the retrograde stars are stripped from GSE's outer disk early in the merger. Despite being selected purely on angular momenta and radial distributions, this simulation reproduces and explains the following phenomena: (i) the triaxial shape of the inner halo, whose major axis is at approximate to 35 degrees to the plane and connects GSE's apocenters; (ii) the Hercules-Aquila Cloud and the Virgo Overdensity, which arise due to apocenter pileup; and (iii) the 2 Gyr lag between the quenching of GSE and the truncation of the age distribution of the in situ halo, which tracks the lag between the first and final GSE pericenters. We make the following predictions: (i) the inner halo has a "double-break" density profile with breaks at both approximate to 15-18 kpc and 30 kpc, coincident with the GSE apocenters; and (ii) the outer halo has retrograde streams awaiting discovery at >30 kpc that contain approximate to 10% of GSE's stars. The retrograde (radial) GSE debris originates from its outer (inner) disk-exploiting this trend, we reconstruct the stellar metallicity gradient of GSE (-0.04 +/- 0.01 dex r(50)(-1)). These simulations imply that GSE delivered approximate to 20% of the Milky Way's present-day dark matter and approximate to 50% of its stellar halo.