Space Telescope Imaging Spectrograph Long-Slit Spectroscopy of the Narrow-Line Region of NGC 4151. II. Physical Conditions along Position Angle 221°

We have examined the physical conditions in the narrow-line region of the well-studied Seyfert galaxy NGC 4151, using long-slit spectra obtained with the Hubble Space Telescope Space Telescope Imaging Spectrograph. The data were taken along a position angle of 221°, centered on the optical nucleus. We have generated photoionization models for a contiguous set of radial zones, out to 2.″3 in projected position to the southwest of the nucleus and 2.″7 to the northeast. Given the uncertainties in the reddening correction, the calculated line ratios successfully matched nearly all the dereddened ratios. We find that the narrow-line region consists of dusty atomic gas photoionized by a power-law continuum that has been modified by transmission through a mix of low- and high-ionization gas, specifically, UV-absorbing and X-ray-absorbing components. The physical characteristics of the absorbers resemble those observed along our line of sight to the nucleus, although the column density of the X-ray absorber is a factor of 10 less than observed. The large inferred covering factor of the absorbing gas is in agreement with the results of our previous study of UV absorption in Seyfert 1 galaxies. We find evidence, specifically the suppression of Lyα, that we are observing the back end of dusty ionized clouds in the region southwest 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 inhomogeneous and can be modeled as consisting of a radiation-bounded component 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 emission-line clouds.