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

TL;DR

This study introduces the Lumen model to explain extreme emission-line ratios in high-redshift galaxies, highlighting the roles of ionization parameters, stellar populations, and chemical abundances.

Contribution

The paper develops and applies the Lumen framework to cosmological simulations, providing new insights into the physical mechanisms behind high-redshift galaxy emission-line ratios.

Findings

High ionization parameters from massive star clusters explain extreme line ratios.

Enhanced nitrogen abundances are necessary for high [NII]/Hα ratios.

Models combining harder spectra, elevated ionization, and nitrogen enrichment match observations.

Abstract

Optical emission-line ratios in star-forming galaxies at $z \sim 3$-8, such as [OIII]/H$\beta$ 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$\alpha$ versus [OIII]/H$\beta$ (N2-BPT) diagram, the [SII]/H$\alpha$ versus [OIII]/H$\beta$ (S2-VO87) diagram, and the [OIII]/[OII] versus ([OII]+[OIII])/H$\beta$ (O32-R23) diagram. We find that $\alpha$-enhancement alone cannot explain the bulk of observations. Moderate offsets emerge from the combined effects of $\alpha$-enhancement, a higher IMF upper-mass cutoff, and AGN contributions. The most extreme [OIII]/H$\beta$ 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$\alpha$ 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$\alpha$ 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.