Timing the Early Assembly of the Milky Way with the H3 Survey
  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], [$\alpha$/Fe],
and orbits from the H3 Survey and Gaia to time the major events in the early
Galaxy. Located beyond the Galactic plane, $1\lesssim |Z|/\rm kpc \lesssim4$,
this sample contains three chemically distinct groups: a low metallicity
population, and low-$\alpha$ and high-$\alpha$ groups at higher metallicity.
The age and orbit distributions of these populations show that: 1) the
high-$\alpha$ group, which includes both disk stars and the in-situ halo, has a
star-formation history independent of eccentricity that abruptly truncated
$8.3\pm0.1$ Gyr ago ($z\simeq1$); 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 ($z\simeq2$); 3) the
low-$\alpha$ 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 $z\approx2$ that merged with the
early disk by $z\approx1$. This merger truncated star formation in the early
high-$\alpha$ disk and perturbed a fraction of that disk onto halo-like orbits.
The merger enabled the formation of a chemically distinct, low-$\alpha$ disk at
$z\lesssim1$. 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.
