Timing the Early Assembly of the Milky Way with the H3 Survey
Abstract
The archaeological record of stars in the Milky Way opens a uniquely detailed window into the early formation and assembly of galaxies. Here we use 11,000 main-sequence turn-off stars with well-measured ages, [Fe/H], [α/Fe], and orbits from the H3 Survey and Gaia to time the major events in the early Galaxy. Located beyond the Galactic plane, 1 |Z|/ kpc 4, this sample contains three chemically distinct groups: a low metallicity population, and low-α and high-α groups at higher metallicity. The age and orbit distributions of these populations show that: 1) the high-α group, which includes both disk stars and the in-situ halo, has a star-formation history independent of eccentricity that abruptly truncated 8.30.1 Gyr ago (z1); 2) the low metallicity population, which we identify as the accreted stellar halo, is on eccentric orbits and its star formation truncated 10.2.+0.2-0.1 Gyr ago (z2); 3) the low-α population is primarily on low eccentricity orbits and the bulk of its stars formed less than 8 Gyr ago. These results suggest a scenario in which the Milky Way accreted a satellite galaxy at z≈2 that merged with the early disk by z≈1. This merger truncated star formation in the early high-α disk and perturbed a fraction of that disk onto halo-like orbits. The merger enabled the formation of a chemically distinct, low-α disk at z1. The lack of any stars on halo-like orbits at younger ages indicates that this event was the last significant disturbance to the Milky Way disk.