Extended high-ionization nuclear emission-line region in the Seyfert galaxy NGC 4051

We present an optical spectroscopic analysis of the well-known Seyfert galaxy NGC 4051. The high-ionization nuclear emission-line region (HINER) traced by [Fe x] λ6374 is found to be spatially extended to a radius of 3″ (≈150 pc) west and southwest from the nucleus; NGC 4051 is the third example that has an extended HINER. The nuclear spectrum shows that the flux of [Fe x] λ6374 is stronger than that of [Fe vII] λ6087 in our observation. This property cannot be interpreted in terms of a simple one-zone photoionization model. To understand what happens in the nuclear region in NGC 4051, we investigate the physical condition of the nuclear emission-line region in detail, using new photo-ionization models in which the following three emission-line components are taken into account: (1) optically thick, ionization-bounded clouds; (2) optically thin, matter-bounded clouds; and (3) a contamination component that emits Hα and Hβ lines. Here the observed extended HINER is considered to be associated with the low-density, matter-bounded clouds. Candidates for the contamination component are the broad-line region, the nuclear star-forming regions, or both. The complexity of the excitation condition found in NGC 4051 can be consistently understood if we take into account these contamination components.