{
  "id": "2004.11384",
  "version": "v1",
  "published": "2020-04-23T18:00:02.000Z",
  "updated": "2020-04-23T18:00:02.000Z",
  "title": "Timing the Early Assembly of the Milky Way with the H3 Survey",
  "authors": [
    "Ana Bonaca",
    "Charlie Conroy",
    "Phillip A. Cargile",
    "Rohan P. Naidu",
    "Benjamin D. Johnson",
    "Dennis Zaritsky",
    "Yuan-Sen Ting",
    "Nelson Caldwell",
    "Jiwon Jesse Han",
    "Pieter van Dokkum"
  ],
  "comment": "submitted to AAS Journals",
  "categories": [
    "astro-ph.GA"
  ],
  "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], [$\\alpha$/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.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2020-04-23T18:00:02.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "milky way",
      "h3 survey",
      "low metallicity population",
      "early assembly",
      "chemically distinct"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 0,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}