Where is the [O III] λ4363 emitting region in active galactic nuclei?

The emission-line flux ratio of [O III] lambda 4363/[O III] lambda 5007 (R-O III) is a useful diagnostic for the ionization mechanism and physical properties of emission-line regions in active galactic nuclei (AGNs). However, it is known that simple photoionization models underpredict the [O III] lambda 4363 intensity, being inconsistent with observations. In this paper we report on several pieces of evidence that a large fraction of the [O III] lambda 4363 emission arises from the dense gas obscured by putative tori : (1) the visibility of high-R-O III regions is correlated to that of broad-line regions, (2) higher R-O III objects show hotter mid-infrared colors, (3) higher R-O III objects show stronger highly ionized emission lines such as [Fe VII] lambda 6087 and [Fe X] lambda 6374, and (4) higher R-O III objects have broader line width of [O III] lambda 4363 normalized by that of [O III] lambda 5007. To estimate how such a dense component contributes to the total emission-line flux, dual-component photoionization model calculations are performed. It is shown that the observed values of R-O III of type 1 AGNs may be explained by introducing a 5%-20% contribution from the dense component, while those of type 2 AGNs may be explained by introducing a 0%-2% contribution. We also discuss the [O III] lambda 4363 emitting regions in LINERs in the framework of our dual-component model.