The well-known correlations of forbidden lines FWHM with the critical density for collisional de-excitation and with the ionization potential mean, that:
i.the noticeable part of forbidden lines emission must arise in a gas of a rather high density n ∼ ncii.the gas velocity, the density and the degree of ionization they all increase toward the centre of the nucleus.
A Study of Three-Dimensional Kinematics of the Narrow-Line Region of the Seyfert Galaxy NGC 4151 Based on Integral Field Spectroscopy