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We compute synthetic optical and ultraviolet (UV) emission-line properties ofgalaxies in a full cosmological framework by coupling, in post-processing,new-generation nebular-emission models with high-resolution, cosmologicalzoom-in simulations of massive galaxies. Our self-consistent modelling accountsfor nebular emission from young stars and accreting black holes (BHs). Weinvestigate which optical- and UV-line diagnostic diagrams can best help todiscern between the main ionizing sources, as traced by the ratio of BHaccretion to star formation rates in model galaxies, over a wide range ofredshifts. At low redshift, simulated star-forming galaxies, galaxies dominatedby active galactic nuclei and composite galaxies are appropriatelydifferentiated by standard selection criteria in the classical[OIII]$\lambda$5007/H$\beta$ versus [NII]$\lambda$6584/H$\alpha$ diagram. Atredshifts $z \gt 1$, however, this optical diagram fails to discriminatebetween active and inactive galaxies at metallicities below $0.5\ Z_\odot$. Torobustly classify the ionizing radiation of such metal-poor galaxies, whichdominate in the early Universe, we confirm 3 previous, and propose 11 noveldiagnostic diagrams based on equivalent widths and luminosity ratios of UVemission lines, such as EW(OIII]$\lambda$1663) versusOIII]$\lambda$1663/HeII$\lambda$1640, CIII]$\lambda$1908/HeII$\lambda$1640versus OIII]$\lambda$1663/HeII$\lambda$1640, andCIV$\lambda$1550/CIII]$\lambda$1908 versus CIII]$\lambda$1908/CII$\lambda$2326.We formulate associated UV selection criteria and discuss some caveats of ourresults (e.g., uncertainties in the modelling of the HeII$\lambda$1640 line).These UV diagnostic diagrams are potentially important for the interpretationof high-quality spectra of very distant galaxies to be gathered bynext-generation telescopes, such as the James Webb Space Telescope.

Synthetic nebular emission from massive galaxies II: ultraviolet-line diagnostics of dominant ionizing sources