Iron is not depleted in high-ionization nuclear emission-line regions of active Galactic nuclei

To examine whether or not high-ionization nuclear emission-line regions (HINERs) in narrow-line regions of active galactic nuclei are dusty, we focus on two high-ionization forbidden emission lines, [Fe VII] lambda 6087 and [Ne V] lambda3426. We perform photoionization model calculations to investigate possible dependences of the flux ratio of [Fe VII] lambda6087/[Ne V]lambda3426 on various gas properties, in order to investigate how useful this flux ratio is to explore the dust abundances in HINERs. Based on our photoionization model calculations, we show that the observed range of the flux ratio of [Fe VII] lambda6087/[Ne V] lambda3426 is consistent with the dust-free models, while it cannot easily be explained by the dusty models. This suggests that iron is not depleted in HINERs, which implies that the HINERs are not dusty. This result is consistent with the idea that the HINERs are located closer than the dust-sublimation radius (i.e., the inner radius of dusty tori) and thus can be hidden by dusty tori when seen from a edge-on view toward the tori, which is also suggested by the AGN-type dependence of the visibility of high-ionization emission lines.