{
  "id": "1806.05680",
  "version": "v1",
  "published": "2018-06-14T18:00:01.000Z",
  "updated": "2018-06-14T18:00:01.000Z",
  "title": "The formation and assembly history of the Milky Way revealed by its globular cluster population",
  "authors": [
    "J. M. Diederik Kruijssen",
    "Joel L. Pfeffer",
    "Marta Reina-Campos",
    "Robert A. Crain",
    "Nate Bastian"
  ],
  "comment": "24 pages (including appendices), 6 figures, 5 tables; accepted by MNRAS (June 12, 2018), originally submitted on March 25, 2018; Figures 4 and 6 show the main results of the paper",
  "categories": [
    "astro-ph.GA"
  ],
  "abstract": "We use the age-metallicity distribution of 96 Galactic globular clusters (GCs) to infer the formation and assembly history of the Milky Way (MW), culminating in the reconstruction of its merger tree. Based on a quantitative comparison of the Galactic GC population to the 25 cosmological zoom-in simulations of MW-mass galaxies in the E-MOSAICS project, which self-consistently model the formation and evolution of GC populations in a cosmological context, we find that the MW assembled quickly for its mass, reaching $\\{25,50\\}\\%$ of its present-day halo mass already at $z=\\{3,1.5\\}$ and half of its present-day stellar mass at $z=1.2$. We reconstruct the MW's merger tree from its GC age-metallicity distribution, inferring the number of mergers as a function of mass ratio and redshift. These statistics place the MW's assembly $\\textit{rate}$ among the 72th-94th percentile of the E-MOSAICS galaxies, whereas its $\\textit{integrated}$ properties (e.g. number of mergers, halo concentration) match the median of the simulations. We conclude that the MW has experienced no major mergers (mass ratios $>$1:4) since $z\\sim4$, sharpening previous limits of $z\\sim2$. We identify three massive satellite progenitors and constrain their mass growth and enrichment histories. Two are proposed to correspond to Sagittarius (few $10^8~{\\rm M}_\\odot$) and Canis Major ($\\sim10^9~{\\rm M}_\\odot$). The third satellite has no known associated relic and was likely accreted between $z=0.6$-$1.3$. We name this enigmatic galaxy $\\textit{Kraken}$ and propose that it is the most massive satellite ($M_*\\sim2\\times10^9~{\\rm M}_\\odot$) ever accreted by the MW. We predict that $\\sim40\\%$ of the Galactic GCs formed ex-situ (in galaxies with masses $M_*=2\\times10^7$-$2\\times10^9~{\\rm M}_\\odot$), with $6\\pm1$ being former nuclear clusters.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2018-06-14T18:00:01.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "globular cluster population",
      "milky way",
      "assembly history",
      "merger tree",
      "galactic gc"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 24,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}