{
  "abstract": "APOGEE and Gaia data suggest the Milky Way’s kinematic structure evolved through three distinct phases: a disordered protogalaxy, which ‘spun up’ into a thick stellar disk, and then ‘cooled down’ to a final cold, thin stellar disk. We use a suite of FIRE-2 cosmological zoom-in simulations of Milky Way-mass galaxies to demonstrate that the same three phases arise in simulations, and we study their physical origin. In our simulations, the early protogalaxy phase occurs when the rate of cool gas (T ≤ 104 K) converting into stars is low, star formation is bursty, and the baryonic mass ‘sloshes’ within the host potential with respect to the center of mass. The gas begins to spin coherently after sloshing ends, followed by the spin-up of young stars. The central potential is least concentrated just prior to gas spin-up. This second, thick disk phase coincides with the highest rate of cool gas converting into stars, though star formation remains bursty. The final transition to the thin disk phase occurs when the inner circumgalactic medium virializes, and is associated with steady star formation and intermediate consumption rates of cool gas converting into stars. Mergers do not appear to play a defining role in driving transitions between the three phases. The condition for thick disk formation appears to be minimal: a stable center of mass motion. Thin disk formation requires more: gas must accrete slowly enough for its angular momentum to mix and become coherent prior to joining the galaxy.",
  "abstract_shown": true,
  "affiliations": [
    "University of California Department of Physics and Astronomy, , Irvine, CA 92697 ,",
    "University of Southern California Department of Physics and Astronomy, , Los Angeles, CA 90089 ,",
    "The University of Texas at Austin Department of Astronomy, , Austin, TX 78712 ,",
    "Cosmic Frontier Center, The University of Texas at Austin , Austin, TX 78712 ,",
    "Weinberg Institute for Theoretical Physics, The University of Texas at Austin , Austin, TX 78712 ,",
    "Center for Astrophysics | Harvard & Smithsonian , 60 Garden St, Cambridge, MA 02138 ,",
    "Northwestern University Department of Physics and Astronomy and CIERA, , 2145 Sheridan Road, Evanston, IL 60208 ,",
    "University of Zurich Department of Astrophysics, , Winterthurerstrasse 190, 8057 Zurich ,",
    "Yale University Department of Astronomy, , New Haven, CT 06520 ,",
    "Pomona College Department of Physics and Astronomy, , Claremont, CA 91711 ,",
    "Carnegie Observatories , 813 Santa Barbara St., Pasadena, CA 91101 ,",
    "School of Physics and Astronomy, Tel Aviv University , Tel Aviv 69978 ,",
    "University of California Department of Physics and Astronomy, , Davis, CA 95616 ,"
  ],
  "article_type": null,
  "arxiv": null,
  "author_total": 12,
  "authors": [
    {
      "affs": [
        0
      ],
      "name": "Olti Myrtaj"
    },
    {
      "affs": [
        0,
        1
      ],
      "name": "James S Bullock",
      "orcid": "0000-0003-4298-5082"
    },
    {
      "affs": [
        2,
        3,
        4
      ],
      "name": "Michael Boylan-Kolchin"
    },
    {
      "affs": [
        5
      ],
      "name": "Vedant Chandra",
      "orcid": "0000-0002-0572-8012"
    },
    {
      "affs": [
        6
      ],
      "name": "Claude-André Faucher-Giguère"
    },
    {
      "affs": [
        7
      ],
      "name": "Robert Feldmann",
      "orcid": "0000-0002-1109-1919"
    },
    {
      "affs": [
        8
      ],
      "name": "Pratik J Gandhi"
    },
    {
      "affs": [
        9
      ],
      "name": "Francisco J Mercado"
    },
    {
      "affs": [
        9,
        10
      ],
      "name": "Jorge Moreno",
      "orcid": "0000-0002-3430-3232"
    },
    {
      "affs": [
        11
      ],
      "name": "Jonathan Stern",
      "orcid": "0000-0002-7541-9565"
    },
    {
      "affs": [
        12
      ],
      "name": "Andrew Wetzel",
      "orcid": "0000-0003-0603-8942"
    },
    {
      "affs": [
        0
      ],
      "name": "Luke Y Xia",
      "orcid": "0009-0002-4568-2423"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/dm5qf33vgpx69.bib",
  "cited_by": 0,
  "doi": "10.1093/mnras/stag1902",
  "fulltext_url": null,
  "id": "dm5qf33vgpx69",
  "issns": [
    "0035-8711",
    "1365-2966"
  ],
  "landing_url": "https://academic.oup.com/mnras/advance-article/doi/10.1093/mnras/stag1902/8907363",
  "language": null,
  "licence": "CC-BY-4.0",
  "licence_label": "CC BY 4.0",
  "licence_source": "article",
  "licence_url": "https://creativecommons.org/licenses/by/4.0/",
  "metadata_licence": null,
  "native_subjects": [
    [
      "openalex",
      "3103|Galaxies: Formation, Evolution, Phenomena"
    ]
  ],
  "openalex": "W7162111922",
  "pdf_url": "https://academic.oup.com/mnras/advance-article-pdf/doi/10.1093/mnras/stag1902/71838615/stag1902.pdf",
  "pmcid": null,
  "pmid": null,
  "published": "2026-10-10",
  "publisher": "Oxford University Press (OUP)",
  "ris": "https://research.draconitic.com/paper/dm5qf33vgpx69.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:18:17Z",
      "native_id": "10.1093/mnras/stag1902",
      "source": "crossref",
      "url": "https://academic.oup.com/mnras/advance-article/doi/10.1093/mnras/stag1902/8907363"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "openalex:3103|Galaxies: Formation, Evolution, Phenomena",
      "subfield": "galaxies"
    }
  ],
  "title": "From protogalaxy through thick and thin: Why did the Milky Way evolve in three kinematic phases?",
  "title_text": "From protogalaxy through thick and thin: Why did the Milky Way evolve in three kinematic phases?",
  "type": "peer-reviewed",
  "updated_at": "2026-10-30T12:18:17Z",
  "url": "https://research.draconitic.com/paper/dm5qf33vgpx69",
  "venue": "Monthly Notices of the Royal Astronomical Society",
  "versions": []
}