Modelling emission lines in star-forming galaxies

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 H II region models with an empirical determination of how the properties of H II 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 ≈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 H II region properties to evolve with redshift, such as the gas density, and cannot be reproduced by H II model grids that only allow the gas metallicity and ionisation parameter to vary.