Seyfert-Type Dependences of Narrow Emission-Line Ratios and Physical Properties of High-Ionization Nuclear Emission-Line Regions in Seyfert Galaxies

In order to examine how narrow emission-line flux ratios depend on theSeyfert type, we compiled various narrow emission-line flux ratios of 355Seyfert galaxies from the literature. We present in this paper that theintensity of the high-ionization emission lines, [Fe VII]6087, [Fe X]6374 and[Ne V]3426, tend to be stronger in Seyfert 1 galaxies than in Seyfert 2galaxies. In addition to these lines, [O III]4363 and [Ne III]3869, whoseionization potentials are not high (< 100 eV), but whose critical densities aresignificantly high (> 10^7 cm^-3), also exhibit the same tendency. On the otherhand, the emission-line flux ratios among low-ionization emission lines do notshow such a tendency. We point out that the most plausible interpretation ofthese results is that the high-ionization emission lines arise mainly fromhighly-ionized, dense gas clouds, which are located very close to nuclei, andthus can be hidden by dusty tori. To examine the physical properties of thesehighly-ionized dense gas clouds, photoionization model calculations wereperformed. As a result, we find that the hydrogen density and the ionizationparameter of these highly-ionized dense gas clouds are constrained to be n_H >10^6 cm^-3 and U > 10^-2, respectively. These lower limits are almostindependent both from the metallicity of gas clouds and from the spectralenergy distribution of the nuclear ionizing radiation.Comment: 32 pages, to appear in Publications of the Astronomical Society of Japa