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

Abstract Optical emission-line ratios in star-forming galaxies at z ~ 3–8, such as [O iii]/Hβ and [O iii]/[O ii], 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 H ii regions in cosmological simulations. We apply Lumen to IllustrisTNG50, validate its predictions at low redshift, and test mechanisms proposed to explain extreme line ratios at z = 3–8. We focus on the [N ii]/Hα vs. [O iii]/Hβ (N2-BPT) diagram, the [S ii]/Hα vs. [O iii]/Hβ (S2-VO87) diagram, and the [O iii]/[O ii] vs. ([O ii]+[O iii])/Hβ (O32–R23) diagram. We find that stellar α-enhancement alone cannot explain the bulk of observations. Moderate offsets arise from the combined effects of stellar α-enhancement and a higher IMF upper-mass cutoff, while AGN may contribute to some of the observed scatter. The most extreme [O iii]/Hβ and [O iii]/[O ii] values require high ionization parameters powered by massive star clusters of ≳ 105–106 M⊙, consistent with recent JWST observations. Reproducing the highest [N ii]/Hα ratios requires enhanced nitrogen abundances. Although gas densities of n ~ 104 cm−3 can boost several diagnostic ratios, they suppress [S ii]/Hα, in tension with current observations. Overall, a model combining harder ionizing spectra, elevated ionization parameters from massive star clusters, and enhanced nitrogen abundances can reproduce all observed high-z line ratios across the N2-BPT, S2-VO87, and O32–R23 diagrams. Based on this, we provide new demarcation lines for star-forming galaxies in the N2-BPT and S2-VO87 diagrams.