{
  "abstract": "Optical emission-line ratios in star-forming galaxies at $z \\sim 3$-8, such as [OIII]/H$β$ and [OIII]/[OII], are strongly offset from those at $z \\sim 0$-2, pointing to more extreme ionization and ISM conditions in the early Universe. To constrain the physical origin of these offsets, we developed Lumen, a framework for modelling nebular emission from spatially distributed HII regions in cosmological simulations. We apply Lumen to IllustrisTNG50, validate its predictions at low redshift, and test a suite of proposed mechanisms for producing extreme line ratios at $z = 3$-8. We focus on the [NII]/H$α$ versus [OIII]/H$β$ (N2-BPT) diagram, the [SII]/H$α$ versus [OIII]/H$β$ (S2-VO87) diagram, and the [OIII]/[OII] versus ([OII]+[OIII])/H$β$ (O32-R23) diagram. We find that $α$-enhancement alone cannot explain the bulk of observations. Moderate offsets emerge from the combined effects of $α$-enhancement, a higher IMF upper-mass cutoff, and AGN contributions. The most extreme [OIII]/H$β$ and [OIII]/[OII] values require high ionization parameters powered by massive star clusters of $\\gtrsim 10^5$-$10^6\\,\\mathrm{M}_\\odot$, consistent with recent JWST observations. Reproducing the highest [NII]/H$α$ ratios additionally requires enhanced nitrogen abundances. Although gas densities of $n \\sim 10^4\\,\\mathrm{cm}^{-3}$ can boost several diagnostic ratios, they suppress [SII]/H$α$ and are therefore in tension with current observations. Overall, models combining harder ionizing spectra, elevated ionization parameters from massive star clusters, and enhanced nitrogen abundances reproduce the observed high-$z$ galaxy population across the N2-BPT, S2-VO87, and O32-R23 diagrams. This successful model also motivates new demarcation lines for star-forming galaxies in the N2-BPT and S2-VO87 diagrams.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2605.06769",
  "author_total": 14,
  "authors": [
    {
      "name": "Lucie Scharré"
    },
    {
      "name": "Michaela Hirschmann"
    },
    {
      "name": "Adèle Plat"
    },
    {
      "name": "Stephane Charlot"
    },
    {
      "name": "Rachel S. Somerville"
    },
    {
      "name": "Emma Curtis-Lake"
    },
    {
      "name": "Gabriella De Lucia"
    },
    {
      "name": "Miroslava Dessauges-Zavadsky"
    },
    {
      "name": "Anna Feltre"
    },
    {
      "name": "Marion Farcy"
    },
    {
      "name": "Pascale Jablonka"
    },
    {
      "name": "Natalia Lahén"
    },
    {
      "name": "Aswin P. Vijayan"
    },
    {
      "name": "Stephen M. Wilkins"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/9zxf1kjhmxcdv.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2605.06769",
  "fulltext_url": null,
  "id": "9zxf1kjhmxcdv",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2605.06769",
  "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/2605.06769",
  "pmcid": null,
  "pmid": null,
  "published": "2026-11-02",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/9zxf1kjhmxcdv.ris",
  "sources": [
    {
      "first_seen": "2026-11-04T15:21:24Z",
      "native_id": "2605.06769",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2605.06769"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.GA",
      "subfield": "galaxies"
    }
  ],
  "title": "Origins of Extreme Emission-Line Ratios in z > 3 Galaxies: Insights from the Lumen Model",
  "title_text": "Origins of Extreme Emission-Line Ratios in z > 3 Galaxies: Insights from the Lumen Model",
  "type": "preprint",
  "updated_at": "2026-11-06T18:21:01Z",
  "url": "https://research.draconitic.com/paper/9zxf1kjhmxcdv",
  "venue": "arXiv",
  "versions": [
    {
      "doi": "10.1093/mnras/stag1729",
      "pid": null,
      "relation": "published-as",
      "title": null,
      "venue": null
    }
  ]
}