{
  "abstract": "Little red dots (LRDs) challenge conventional models of active galactic nuclei. At rest-optical-to-near-infrared (IR) wavelengths, these compact extragalactic objects show blackbody-like continuum emission and spectral features reminiscent of stars, motivating models with an optically thick atmosphere at T eff ∼ 4000–5000 K. We develop (and publicly release) a synthetic spectral library of optically thick atmospheres with gas conditions tailored for LRDs, parameterized by effective temperature T eff and surface gravity g . Given the uncertain dynamical structure of LRDs, we interpret g mainly as a proxy for the photospheric density ρ ph . We show that blackbodies are only crude approximations to the emission from LRD-like atmospheres. Spectral features are abundant, many of which are sensitive diagnostics of photospheric density, including the overall curvature of the continuum, the rest-1.6 μ m “kink” from H ⁻ opacity, and the Ca II triplet (CaT) absorption at rest-8500 Å. When compared against a local LRD, the Egg , all three features are consistent with a low photospheric density ρ ph ∼ 10 ⁻¹¹ g cm ⁻³ ( g ∼ 10 ⁻³ cm s ⁻² in our library); although CaT alone admits another higher-density solution. This low ρ ph directly results from our radiative transfer modeling; with the additional assumption that the CaT line width traces turbulent support at the continuum photosphere in a spherical geometry, we infer a mass within the photosphere (black hole plus gas) of ∼10 ⁴ M ⊙ , with an Eddington ratio λ Edd ≳ 20. For higher-redshift LRDs, we advocate for rest-near-IR spectroscopic surveys and high-resolution spectra of potential absorption lines as a test of the optically thick atmosphere scenario and as a unique probe of the central engine mass.",
  "abstract_shown": true,
  "affiliations": [
    "Princeton University",
    "Flatiron Institute",
    "Tsinghua University"
  ],
  "article_type": null,
  "arxiv": null,
  "author_total": 6,
  "authors": [
    {
      "affs": [
        0
      ],
      "name": "Hanpu 翰溥 Liu 刘",
      "orcid": "0000-0003-2488-4667"
    },
    {
      "affs": [
        1
      ],
      "name": "Yan-Fei 燕飞 Jiang 姜",
      "orcid": "0000-0002-2624-3399"
    },
    {
      "affs": [
        0
      ],
      "name": "Eliot Quataert",
      "orcid": "0000-0001-9185-5044"
    },
    {
      "affs": [
        0
      ],
      "name": "Jenny E. Greene",
      "orcid": "0000-0002-5612-3427"
    },
    {
      "affs": [
        0
      ],
      "name": "Yilun 逸伦 Ma 马",
      "orcid": "0000-0002-0463-9528"
    },
    {
      "affs": [
        2
      ],
      "name": "Xiaojing Lin",
      "orcid": "0000-0001-6052-4234"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/a264p8cbm182f.bib",
  "cited_by": 0,
  "doi": "10.3847/1538-4357/ae9d72",
  "fulltext_url": null,
  "id": "a264p8cbm182f",
  "issns": [
    "0004-637X",
    "1538-4357"
  ],
  "landing_url": "https://iopscience.iop.org/article/10.3847/1538-4357/ae9d72",
  "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": "W7221135873",
  "pdf_url": "https://iopscience.iop.org/article/10.3847/1538-4357/ae9d72/pdf",
  "pmcid": null,
  "pmid": null,
  "published": "2026-10-09",
  "publisher": "American Astronomical Society",
  "ris": "https://research.draconitic.com/paper/a264p8cbm182f.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:18:11Z",
      "native_id": "10.3847/1538-4357/ae9d72",
      "source": "crossref",
      "url": "https://iopscience.iop.org/article/10.3847/1538-4357/ae9d72"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "openalex:3103|Galaxies: Formation, Evolution, Phenomena",
      "subfield": "galaxies"
    }
  ],
  "title": "Synthetic Spectral Library of Optically Thick Atmospheres for Little Red Dots",
  "title_text": "Synthetic Spectral Library of Optically Thick Atmospheres for Little Red Dots",
  "type": "peer-reviewed",
  "updated_at": "2026-10-30T12:18:11Z",
  "url": "https://research.draconitic.com/paper/a264p8cbm182f",
  "venue": "The Astrophysical Journal",
  "versions": []
}