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

Abstract

Abstract 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]$ \lambda6087$, [Fe X]$ \lambda6374$, and [Ne V]$ \lambda3426$, tend to be stronger in Seyfert 1 galaxies than in Seyfert 2 galaxies. In addition to these lines, [O III]$ \lambda4363$ and [Ne III]$ \lambda3869$, whose ionization potentials are not high ($ < 100 \,\mathrm{eV}$), but whose critical densities are significantly high ($ \gtrsim 10^7 \,\mathrm{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_\mathrm{H}> 10^6 \,\mathrm{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.