Abstract
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 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.
Highlights
The energetic photons produced predominantly by hot, young stars can ionise the gas close to them, making HII regions
As we show in Appendix A, the original GALFORM emission line model gives a poor reproduction of the local BPT diagram
The evolution in the model BPT diagram is driven by a combination of an increase in the luminosity of galaxies selected with increasing redshift, an increase in their ionisation parameter with redshift and by allowing the gas density to vary, which alters the shape of the locus of the BPT region
Summary
The energetic photons produced predominantly by hot, young stars can ionise the gas close to them, making HII regions. Orsi et al reduced the HII model grid from two parameters, gas metallicity and ionisation parameter, to one by making the ionisation parameter a function of the gas metallicity This model reproduces the local BPT diagram, but does not evolve with redshift (see Appendix B). Hirschmann et al included the contribution to emission lines from young stars, accretion onto supermassive black holes and older stellar populations that produce ionising photons (post asymptotic giant branch stars) With this sample of galaxies, Hirschmann et al were able to reproduce the local BPT diagram and its evolution for star forming galaxies. We use a different model to connect the global properties of the star forming gas in a GALFORM galaxy to the properties of an HII region than was used by Hirschmann et al We do not consider emission lines resulting from accretion onto supermassive black holes, leaving this for future work. In Appendix C we show the variation in the grids of emission line luminosities returned by two current HII region models
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