Synthetic nebular emission from massive galaxies II: ultraviolet-line
  diagnostics of dominant ionizing sources
  We compute synthetic optical and ultraviolet (UV) emission-line properties of
galaxies in a full cosmological framework by coupling, in post-processing,
new-generation nebular-emission models with high-resolution, cosmological
zoom-in simulations of massive galaxies. Our self-consistent modelling accounts
for nebular emission from young stars and accreting black holes (BHs). We
investigate which optical- and UV-line diagnostic diagrams can best help to
discern between the main ionizing sources, as traced by the ratio of BH
accretion to star formation rates in model galaxies, over a wide range of
redshifts. At low redshift, simulated star-forming galaxies, galaxies dominated
by active galactic nuclei and composite galaxies are appropriately
differentiated by standard selection criteria in the classical
[OIII]$\lambda$5007/H$\beta$ versus [NII]$\lambda$6584/H$\alpha$ diagram. At
redshifts $z \gt 1$, however, this optical diagram fails to discriminate
between active and inactive galaxies at metallicities below $0.5\ Z_\odot$. To
robustly classify the ionizing radiation of such metal-poor galaxies, which
dominate in the early Universe, we confirm 3 previous, and propose 11 novel
diagnostic diagrams based on equivalent widths and luminosity ratios of UV
emission lines, such as EW(OIII]$\lambda$1663) versus
OIII]$\lambda$1663/HeII$\lambda$1640, CIII]$\lambda$1908/HeII$\lambda$1640
versus OIII]$\lambda$1663/HeII$\lambda$1640, and
CIV$\lambda$1550/CIII]$\lambda$1908 versus CIII]$\lambda$1908/CII$\lambda$2326.
We formulate associated UV selection criteria and discuss some caveats of our
results (e.g., uncertainties in the modelling of the HeII$\lambda$1640 line).
These UV diagnostic diagrams are potentially important for the interpretation
of high-quality spectra of very distant galaxies to be gathered by
next-generation telescopes, such as the James Webb Space Telescope.
