Ultraviolet diagnostics for the emission-line gas in active galaxies

Optical diagnostic diagrams are frequently ambiguous as tests of the photoionization or fast-shock models of the narrow-line regions of active galaxies. Here, we present a set of UV line ratio diagrams that can discriminate between pure shock and photoionization modes of excitation and can, to some extent, also discriminate shocks with ionized precursors from photoionization. These diagrams use relatively bright emission lines and reddening-insensitive ratios and provide a practical observational test for separating the excitation mechanisms of the narrow-line regions of active galaxies. The most useful diagrams are those involving the various ionization stages of carbon, ([O III] lambda 5007/H beta) versus (C IV lambda 1549/ He II lambda 1640) and the purely UV ratio pair (C II] lambda 2326/C III] lambda 1909) versus (C IV lambda 1549/C III] lambda 1909). Temperature-sensitive FUV lines, C III lambda 977 and N III lambda 991, also provide good discriminants. The models are compared to observations of nearby AGNs and also to high-redshift objects where the UV lines are shifted into the optical.