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