A new dual-component photoionization model for the narrow emission line regions in active galactic nuclei

Having found that type 1 Seyfert nuclei have excess [Fe VII] λ6087 emission with respect to type 2 nuclei, Murayama & Taniguchi have proposed that the high-ionization nuclear emission line region (HINER) traced by the [Fe VII] λ6087 emission resides in the inner wall of dusty tori. The covering factor of the torus is usually large (e.g., ∼0.9). Further, electron density in the tori (e.g., ∼10⁷-10⁸ cm^-3) is considered to be significantly higher than that (e.g., ∼10³-10⁴ cm^-3) in the narrow-line region (NLR). Therefore, it is expected that the torus emission contributes to the majority of the higher ionization emission lines. Taking this HINER component into account, we have constructed new dual-component (i.e., a typical NLR with a HINER torus) photoionization models. Comparison of our model with the observations shows that if the torus emission contributes ∼10% of the NLR emission, our dual-component model can explain the observed high [Fe VII] λ6087/[O III] λ5007 intensity ratios of the Seyfert 1 nuclei without invoking any unusual assumptions (e.g., the overabundance of iron).