Where is the [O III]4363 emitting region in active galactic nuclei?

Abstract

The emission-line flux ratio of [O III]4363/[O III]5007 (R_OIII)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]4363 intensity, being inconsistent with observations. In this paper, we report on several pieces of evidence that a large fraction of the [O III]4363 emission arises from the dense gas obscured by putative tori: (1) the visibility of high-R_OIII regions is correlated to that of broad-line regions, (2) higher-R_OIII objects show hotter mid-infrared colors, (3) higher-R_OIII objects show stronger highly-ionized emission lines such as [Fe VII]6087 and [Fe X]6374, and (4) higher-R_OIII objects have broader line width of [O III]4363 normalized by that of [O III]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_OIII 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]4363 emitting regions in LINERs in the framework of our dual-component model.