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 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.
