Modelling the nebular emission from primeval to present-day star-forming galaxies

TL;DR

This paper introduces a comprehensive model for nebular emission in star-forming galaxies across cosmic time, integrating stellar synthesis and photoionization to interpret emission lines and chemical compositions.

Contribution

The model uniquely combines stellar population synthesis with photoionization codes, accounting for element depletion and dust, to accurately interpret nebular emission across different galaxy types and epochs.

Findings

Ultraviolet lines are more sensitive to physical parameters than optical lines.

Standard ionic fraction and C/O ratio formulas are valid at near-solar metallicities.

Fully self-consistent models are necessary for chemically young, low-metallicity galaxies.

Abstract

We present a new model of the nebular emission from star-forming galaxies in a wide range of chemical compositions, appropriate to interpret observations of galaxies at all cosmic epochs. The model relies on the combination of state-of-the-art stellar population synthesis and photoionization codes to describe the ensemble of HII regions and the diffuse gas ionized by young stars in a galaxy. A main feature of this model is the self-consistent yet versatile treatment of element abundances and depletion onto dust grains, which allows one to relate the observed nebular emission from a galaxy to both gas-phase and dust-phase metal enrichment. We show that this model can account for the rest-frame ultraviolet and optical emission-line properties of galaxies at different redshifts and find that ultraviolet emission lines are more sensitive than optical ones to parameters such as C/O abundance ratio, hydrogen gas density, dust-to-metal mass ratio and upper cutoff of the stellar initial mass function. We also find that, for gas-phase metallicities around solar to slightly sub-solar, widely used formulae to constrain oxygen ionic fractions and the C/O ratio from ultraviolet and optical emission-line luminosities are reasonable faithful. However, the recipes break down at non-solar metallicities, making them inappropriate to study chemically young galaxies. In such cases, a fully self-consistent model of the kind presented in this paper is required to interpret the observed nebular emission.