A New Dual-Component Photoionization Model for the Narrow Emission-Line Regions in Active Galactic Nuclei

Abstract

Having found that type 1 Seyfert nuclei have excess [FeVII]6087 emission with respect to type 2s, Murayama & Taniguchi have proposed that the high-ionization nuclear emission-line region (HINER) traced by the [FeVII]6087 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., ~ 10^7-10^8 cm-3) is considered to be higher significantly than that (e.g., ~ 10^3-10^4 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 show that that if the torus emission contributes ~ 10 % of the NLR emission, our dual-component model can explain the observed high [FeVII]6087/[OIII]5007 intensity ratios of the Seyfert 1s without invoking any unusual assumptions (e.g., the overabundance of iron).