Space Telescope Imaging Spectrograph long-slit spectroscopy of the narrow-line region of NGC 4151. II. Physical conditions along position angle 221 degrees

We have examined the physical conditions in the narrow-line region of the w ell-studied Seyfert galaxy NGC 4151, using long-slit spectra obtained with the Hubble Space Telescope Space Telescope Imaging Spectrograph. The data w ere taken along a position angle of 221 degrees, centered on the optical nu cleus. We have generated photoionization models for a contiguous set of rad ial zones, out to 2." 3 in projected position to the southwest of the nucle us and 2." 7 to the northeast. Given the uncertainties in the reddening cor rection, the calculated line ratios successfully matched nearly all the der eddened ratios. We find that the narrow-line region consists of dusty atomi c gas photoionized by a power-law continuum that has been modified by trans mission through a mix of low- and high-ionization gas, specifically, UV-abs orbing and X-ray-absorbing components. The physical characteristics of the absorbers resemble those observed along our line of sight to the nucleus, a lthough the column density of the X-ray absorber is a factor of 10 less tha n observed. The large inferred covering factor of the absorbing gas is in a greement with the results of our previous study of UV absorption in Seyfert I galaxies. We find evidence, specifically the suppression of Ly alpha, th at we are observing the back end of dusty ionized clouds in the region sout hwest of the nucleus. Since these clouds are blueshifted, this supports the interpretation of the cloud kinematics as being due to radial outflow from the nucleus. We find that the narrow-line gas at each radial position is i nhomogeneous and can be modeled as consisting of a radiation-bounded compon ent and a more tenuous, matter-bounded component. The density of the narrow -line gas drops with increasing radial distance, which confirms our earlier results and may be a result of the expansion of radially outflowing emissi on-line clouds.