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UV DIAGNOSTICS FOR THE EMISSION LINE GAS IN ACTIVE GALAXIES

Optical diagnostic diagrams are frequently ambiguous as a test 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 which can discriminate between pure shock and photoionization modes of excitation, and 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] �5007/H�) vs. (C IV �1549/He II �1640) and the purely UV ratio pair (C II] �2326/C III] �1909) vs. (C IV �1549/C III] �1909). Temperature sensitive FUV lines C III �977 and N III �991 also provide good discriminants. The models are compared to observations of nearby AGN, and also to high redshift objects where the UV lines are shifted into the optical.