Origins of Extreme Emission-Line Ratios in z > 3 Galaxies: Insights from the Lumen Model

@misc{scharre2026origins,
  title = {{Origins of Extreme Emission-Line Ratios in z > 3 Galaxies: Insights from the Lumen Model}},
  author = {Lucie Scharré and Michaela Hirschmann and Adèle Plat and Stephane Charlot and Rachel S. Somerville and Emma Curtis-Lake and Gabriella De Lucia and Miroslava Dessauges-Zavadsky and Anna Feltre and Marion Farcy and Pascale Jablonka and Natalia Lahén and Aswin P. Vijayan and Stephen M. Wilkins},
  howpublished = {arXiv},
  year = {2026},
  month = nov,
  doi = {10.48550/arxiv.2605.06769},
  eprint = {2605.06769},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2605.06769},
  note = {CC BY 4.0},
}

TY  - UNPB
TI  - Origins of Extreme Emission-Line Ratios in z > 3 Galaxies: Insights from the Lumen Model
AU  - Lucie Scharré
AU  - Michaela Hirschmann
AU  - Adèle Plat
AU  - Stephane Charlot
AU  - Rachel S. Somerville
AU  - Emma Curtis-Lake
AU  - Gabriella De Lucia
AU  - Miroslava Dessauges-Zavadsky
AU  - Anna Feltre
AU  - Marion Farcy
AU  - Pascale Jablonka
AU  - Natalia Lahén
AU  - Aswin P. Vijayan
AU  - Stephen M. Wilkins
PY  - 2026
DA  - 2026/11/02
PB  - arXiv
DO  - 10.48550/arxiv.2605.06769
UR  - https://arxiv.org/abs/2605.06769
AB  - Optical emission-line ratios in star-forming galaxies at z ∼ 3-8, such as [OIII]/Hβ and [OIII]/[OII], are strongly offset from those at z ∼ 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]/Hα versus [OIII]/Hβ (N2-BPT) diagram, the [SII]/Hα versus [OIII]/Hβ (S2-VO87) diagram, and the [OIII]/[OII] versus ([OII]+[OIII])/Hβ (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]/Hβ and [OIII]/[OII] values require high ionization parameters powered by massive star clusters of ≳ 10^5-10^6 M_⊙, consistent with recent JWST observations. Reproducing the highest [NII]/Hα ratios additionally requires enhanced nitrogen abundances. Although gas densities of n ∼ 10^4 cm^−3 can boost several diagnostic ratios, they suppress [SII]/Hα 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.
ER  -