Ultraviolet Diagnostics for the Emission‐Line Gas in Active Galaxies

Abstract

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 rela-tively 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 dia-grams are those involving the various ionization stages of carbon, ([O III] j5007/Hb) versus (C IV j1549/ He II j1640) and the purely UV ratio pair (C II] j2326/C III] j1909) versus (C IV j1549/C III] j1909). Temperature-sensitive FUV lines, C III j977 and N III j991, 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.