{
  "abstract": "The Milky Way’s stellar halo preserves a record of its mass assembly history, encoding accretion events in its structural properties. Among the most prominent of these features is a strong break in the halo density profile at ≈20-30 kpc, long attributed to the apocenter pile-up of stars from the Gaia-Enceladus/Sausage merger. However, whether this interpretation is consistent with state-of-the-art cosmological simulations remains unclear. In this work, we compare the Milky Way’s measured stellar halo density profile between galactocentric radii of 1–100 kpc to those of Milky Way-mass galaxies from the FIRE-2 and Auriga cosmological zoom-in simulation suites, spanning a total of 24 simulated galaxies. We find that simulated halo profiles are significantly steeper than Milky Way measurements within ≈15 kpc, and that profile breaks are rare in simulations and never as strong as the Milky Way’s. Of the three galaxies with statistically significant breaks, only one exhibits a break attributable to apocenter pile-up of an accreted merger remnant. Decomposing the simulated profiles, we find that the accreted halo profiles are broadly consistent with Milky Way measurements, while the in-situ halo dominates within ≈20 kpc and drives the discrepancy. These results raise a compelling question: where is the Milky Way’s in-situ halo? The origin of this tension may reflect systematic biases in current halo measurements, an overproduction of early spheroidal star formation in cosmological simulations, or more fundamental differences in the underlying physics (e.g., dark matter) governing the assembly and structure of the inner Galaxy.",
  "abstract_shown": true,
  "affiliations": [
    "University of Pennsylvania Department of Physics, and Astronomy, , 209 S 33rd Street, Philadelphia, PA 19104 ,",
    "Lund University Lund Observatory, Division of Astrophysics, Department of Physics, , SE-221 00 Lund ,",
    "Carleton College Department of Physics and Astronomy, , 1 North College St., Northfield, MN 55057 ,",
    "The University of Texas at Austin Department of Astronomy, , Austin, TX 78712 ,",
    "Cosmic Frontier Center, The University of Texas at Austin , Austin, TX 78712 ,",
    "Graduate Center, City University of New York , 365 5th Avenue, New York, NY 10016 ,"
  ],
  "article_type": null,
  "arxiv": null,
  "author_total": 6,
  "authors": [
    {
      "affs": [
        0
      ],
      "name": "Madeline Lucey",
      "orcid": "0000-0001-7297-8508"
    },
    {
      "affs": [
        0
      ],
      "name": "Robyn Sanderson",
      "orcid": "0000-0003-3939-3297"
    },
    {
      "affs": [
        1
      ],
      "name": "Alexander H Riley",
      "orcid": "0000-0001-5805-5766"
    },
    {
      "affs": [
        2
      ],
      "name": "Andreia Carrillo",
      "orcid": "0000-0002-5786-0787"
    },
    {
      "affs": [
        3,
        4
      ],
      "name": "Jenna Samuel",
      "orcid": "0000-0002-8429-4100"
    },
    {
      "affs": [
        5
      ],
      "name": "Emma Tasso",
      "orcid": "0009-0003-4196-8659"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/aw74335k25133.bib",
  "cited_by": 0,
  "doi": "10.1093/mnras/stag2111",
  "fulltext_url": null,
  "id": "aw74335k25133",
  "issns": [
    "0035-8711",
    "1365-2966"
  ],
  "landing_url": "https://academic.oup.com/mnras/advance-article/doi/10.1093/mnras/stag2111/9012723",
  "language": null,
  "licence": "CC-BY-4.0",
  "licence_label": "CC BY 4.0",
  "licence_source": "journal",
  "licence_url": "https://creativecommons.org/licenses/by/4.0/",
  "metadata_licence": null,
  "native_subjects": [],
  "openalex": null,
  "pdf_url": "https://academic.oup.com/mnras/advance-article-pdf/doi/10.1093/mnras/stag2111/73037074/stag2111.pdf",
  "pmcid": null,
  "pmid": null,
  "published": "2026-11-04",
  "publisher": "Oxford University Press (OUP)",
  "ris": "https://research.draconitic.com/paper/aw74335k25133.ris",
  "sources": [
    {
      "first_seen": "2026-11-05T00:20:45Z",
      "native_id": "10.1093/mnras/stag2111",
      "source": "crossref",
      "url": "https://academic.oup.com/mnras/advance-article/doi/10.1093/mnras/stag2111/9012723"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "journal-openalex:3103|Stellar, planetary, and galactic studies",
      "subfield": "stellar"
    }
  ],
  "title": "The Milky Way’s Missing In-Situ Halo",
  "title_text": "The Milky Way’s Missing In-Situ Halo",
  "type": "peer-reviewed",
  "updated_at": "2026-11-06T00:21:01Z",
  "url": "https://research.draconitic.com/paper/aw74335k25133",
  "venue": "Monthly Notices of the Royal Astronomical Society",
  "versions": []
}