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 [O III]λ5007/H β versus [N II]λ6584/H α diagram. At redshifts z ≳ 1, however, this optical diagram fails to discriminate between active and inactive galaxies at metallicities below 0.5Z. 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(O III] λ1663) versus OIII] λ1663/He II λ1640, C III] λ1908/He II λ1640 versus OIII] λ1663/He II λ1640, and CIV λ1550/C III] λ1908 versus C III] λ1908/C II] λ2326.We formulate associated UV selection criteria and discuss some caveats of our results (e.g. uncertainties in the modelling of the He II λ1640 line). TheseUVdiagnostic 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.