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Session 90 - High Energy Phenomena in AGNs.
Oral session, Friday, January 09
Georgetown,
The excitation mechanism of the narrow line regions (NLR) of active galaxies is commonly thought to be AGN-photoionization. Shocks provide an alternative mechanism based on the input of mechanical energy and are important in cases where the radio plasma or galactic scale outflows interact with the host galaxy ISM. High velocity shocks generate a powerful local UV radiation field which can ionize the gas and emit a highly excited emission spectrum like that observed in the NLR. The UV emission line spectrum is found to be more sensitive to the excitation mechanism than the standard optical diagnostics. 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, [OIII] \lambda5007/H\beta vs. CIV \lambda1549/HeII \lambda1640 and the purely UV ratio pair CII] \lambda2326/CIII] \lambda1909 vs. CIV \lambda1549/CIII] \lambda1909. Temperature sensitive FUV lines CIII \lambda977\ and NIII \lambda991 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.
