{
  "abstract": "Understanding the internal structure of H II regions is fundamental for constraining star formation processes in galaxies. We investigated how the filling factor (FF) relates to luminosity, size, electron density, and H$α$ equivalent width (EW(H$α$)) in H II regions, and explored its connection with polycyclic aromatic hydrocarbon (PAH)-to-dust emission as a possible tracer of evolutionary stages. We analyzed 622 H II regions in NGC~628, combining 475 regions from SIGNALS and 147 from PHANGS-MUSE. We derived their luminosities, emission-line fluxes, radii, electron densities, and FF, and used PHANGS-JWST/MIRI imaging to quantify the PAH-to-dust ratio $R_{\\rm PAH}$ from the 7.7, 11.3, and 21~$μ$m bands. Higher FF and EW(H$α$) values are found in luminous regions, whereas more extended regions with lower EW(H$α$) exhibit lower FF. We show that the H II region radius definition significantly affects the $L_{\\rm Hα}$--$R$ relation. Low-luminosity compact H II regions appear to mark a transition from cluster-powered regions to nebulae ionized by single massive stars, around $\\log(L_{\\rm Hα}) \\sim 37~{\\rm erg \\ s^{-1}}$. The PAH-to-dust ratio correlates with the volumetric H$α$ luminosity density, $L_{\\rm Hα}/R^3$, with a transition around $\\log(L_{\\rm Hα}/R^3) \\sim 32~{\\rm erg \\ s^{-1}}$, corresponding to $\\log({\\rm FF}) \\approx -4.4$. Regions with lower FF exhibit higher $R_{\\rm PAH}$, suggesting less efficient PAH processing in more porous structures. These results are consistent with an evolutionary scenario in which FF decreases with stellar cluster age as giant H II regions evolve toward fainter and more extended states. The volumetric H$α$ luminosity density reduces covariance between $L_{\\rm Hα}$ and $R$ induced by region-definition methods, enabling more consistent cross-catalog comparisons.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2609.00902",
  "author_total": 10,
  "authors": [
    {
      "name": "Lorena Aragüete Riesco"
    },
    {
      "name": "José M. Vílchez"
    },
    {
      "name": "Salvador Duarte Puertas"
    },
    {
      "name": "Laurie Rousseau-Nepton"
    },
    {
      "name": "Carmelle Robert"
    },
    {
      "name": "René Pierre Martin"
    },
    {
      "name": "Philippe Amram"
    },
    {
      "name": "Robert Kennicutt Jr."
    },
    {
      "name": "Christophe Morisset"
    },
    {
      "name": "Ray Garner III"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/ad6wdxt1akf0z.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2609.00902",
  "fulltext_url": null,
  "id": "ad6wdxt1akf0z",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2609.00902",
  "language": null,
  "licence": "CC-BY-4.0",
  "licence_label": "CC BY 4.0",
  "licence_source": "article",
  "licence_url": "http://creativecommons.org/licenses/by/4.0/",
  "metadata_licence": "CC0-1.0",
  "native_subjects": [
    [
      "arxiv",
      "astro-ph.GA"
    ]
  ],
  "openalex": null,
  "pdf_url": "https://arxiv.org/pdf/2609.00902",
  "pmcid": null,
  "pmid": null,
  "published": "2026-09-01",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/ad6wdxt1akf0z.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:18:00Z",
      "native_id": "2609.00902",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2609.00902"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.GA",
      "subfield": "galaxies"
    }
  ],
  "title": "H II region filling factors in NGC 628: Luminosity-size relation and connection with polycyclic aromatic hydrocarbon emission",
  "title_text": "H II region filling factors in NGC 628: Luminosity-size relation and connection with polycyclic aromatic hydrocarbon emission",
  "type": "preprint",
  "updated_at": "2026-10-30T12:18:00Z",
  "url": "https://research.draconitic.com/paper/ad6wdxt1akf0z",
  "venue": "arXiv",
  "versions": [
    {
      "doi": "10.1051/0004-6361/202557698",
      "pid": null,
      "relation": "published-as",
      "title": null,
      "venue": null
    }
  ]
}