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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 the Seyfert type, we compiled various narrow emission-line flux ratios of 355 Seyfert galaxies from the literature. We present in this paper that the intensity 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 2 galaxies. In addition to these lines, [O III]4363 and [Ne III]3869, whose ionization potentials are not high (< 100 eV), but whose critical densities are significantly high (> 10^7 cm^-3), also exhibit the same tendency. On the other hand, the emission-line flux ratios among low-ionization emission lines do not show such a tendency. We point out that the most plausible interpretation of these results is that the high-ionization emission lines arise mainly from highly-ionized, dense gas clouds, which are located very close to nuclei, and thus can be hidden by dusty tori. To examine the physical properties of these highly-ionized dense gas clouds, photoionization model calculations were performed. As a result, we find that the hydrogen density and the ionization parameter 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 almost independent both from the metallicity of gas clouds and from the spectral energy distribution of the nuclear ionizing radiation.