We report on a study of the forbidden emission-line spectrum of the nearby Seyfert 1.5 galaxy NGC 4151 based on the high-resolution (R ~ 45,000) optical spectrum obtained using the High Dispersion Spectrograph on the Subaru Telescope. The profile parameters, such as the emission-line widths, velocity shifts from the recession velocity of the host galaxy, and asymmetry indexes, for emission lines, including very faint ones such as [Ar IV] λλ4712, 4740 and [Fe VI] λλ5631, 5677, are investigated. Statistically significant correlations between the measured profile parameters and the critical densities of transitions are found, while there are no meaningful correlations between the profile parameters and the ionization potentials of ions. By comparing the results with photoionization model calculations, we remark that a simple power-law distribution of the gas density that is independent of the radius from the nucleus cannot explain the observed correlation between the emission-line widths and the critical densities of the transitions. Taking into account the additional dense gas component expected to exist in the innermost area of the narrow-line regions, the observed correlations between the emission-line width and the critical density of the transitions can be understood since high critical density emission lines can arise at such relatively inner regions even if their ionization potentials are low. The observed correlation between the blueshift of the emission lines and the critical densities of the ions is also explained if such dense gas clouds located closer to the nucleus have larger outflowing velocities.
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