Modelling emission lines in star forming galaxies

TL;DR

This paper introduces a new model for emission line luminosities in star forming galaxies, successfully reproducing observed line ratios and their evolution with redshift, highlighting the importance of evolving HII region properties.

Contribution

The paper presents a novel model combining HII region grids with empirical galaxy property relations, improving the understanding of emission line evolution in galaxies.

Findings

Accurately reproduces star-forming galaxy line ratios on diagnostic diagrams.

Shows evolution of emission line ratios with redshift, matching observations at z=1.6.

Predicts higher gas densities and ionisation parameters in high-redshift galaxies.

Abstract

We present a new model to compute the luminosity of emission lines in star forming galaxies and apply this in the semi-analytical galaxy formation code GALFORM. The model combines a pre-computed grid of HII region models with an empirical determination of how the properties of HII regions depend on the macroscopic properties of galaxies based on observations of local galaxies. The new model gives a very good reproduction of the locus of star-forming galaxies on standard line ratio diagnostic diagrams. The new model shows evolution in the locus of star forming galaxies with redshift on this line ratio diagram, with a good match to the observed line ratios at $z=1.6$. The model galaxies at high redshift have gas densities and ionisation parameters that are predicted to be $\approx 2-3$ times higher than in local star forming galaxies, which is partly driven by the changing selection with redshift to mimic the observational selection. Our results suggest that the observed evolution in emission line ratios requires other HII region properties to evolve with redshift, such as the gas density, and cannot be reproduced by HII model grids that only allow the gas metallicity and ionisation parameter to vary.