Extended High-Ionization Nuclear Emission-Line Region in the Seyfert Galaxy NGC 4051

摘要

We present an optical spectroscopic analysis of the well-known Seyfert galaxyNGC 4051. The high-ionization nuclear emission-line region (HINER) traced by[Fe X]6374 is found to be spatially extended to a radius of 3a rcseconds (150pc) west and southwest from the nucleus; NGC 4051 is the third example whichhas an extended HINER. The nuclear spectrum shows that the flux of [Fe X]6374 is stronger than thatof [Fe VII] 6087 in our observation. This property cannot be interpreted interms of a simple one-zone photoionization model. In order to understand what happens in the nuclear region in NGC 4051, weinvestigate the physical condition of the nuclear emission-line region indetail using new photoionization models in which the following threeemission-line components are taken into account; (1) optically thick,ionization-bounded clouds; (2) optically thin, matter-bounded clouds; and (3) acontamination component which emits H$\alpha$ and H$\beta$ lines. Here theobserved extended HINER is considered to be associated with the low-density,matter-bounded clouds. Candidates of the contamination component are either thebroad-line region (BLR) or nuclear star forming regions or both. The complexityof the excitation condition found in NGC 4051 can be consistently understood ifwe take account of these contamination components.