THE EXTENDED NARROW-LINE REGION OF NGC-4151 .2. SPATIAL VARIATIONS OF THE EMISSION-LINE INTENSITIES

We present high quality long-slit spectra of the extended narrow line region (ENLR) near the nucleus of NGC 4151. We have used these data to map the spatial variations of the emission line intensities along three slit positions: one passing radially through the nucleus along the main NE-SW axis of the ENLR and two across this axis at offsets centred on bright knots 6'' and 11'' to the SW, respectively. We find that, with the exception of some isolated H II regions, all the emitting regions sampled by our spectra have line intensities characteristic of photoionization by an AGN-like continuum source. Along the radial axis, there is a clear discontinuity at the boundary between the nuclear narrow line region and the ENLR, where a factor similar to 10 drop in gas density coincides with a corresponding increase in the ionization level. Beyond this point there is an overall decline in ionization along the ENLR, while the gas density remains constant. The radial ionization structure is thus broadly consistent with geometrical dilution of a centrally located ionizing continuum source, presumably the active galactic nucleus. Taken together with the kinematics and morphology, these results strongly support the hypothesis that the ENLR arises from photoionization of gas in the galactic disk by an anisotropic radiation field produced by the active nucleus. Strong lateral ionization gradients also occur across the bright knots, but there is evidence that environmental factors are important and so it is unclear whether these trace an azimuthal variation in the radiation field. The morphology of the ENLR and its misalignment with the nuclear radio source do not conform well to the simple obscuring torus model for Seyfert galaxies. The only plausible geometry which can simultaneously account for both the morphology and misalignment is one in which the galactic disk is partially illuminated by ionizing radiation escaping from the nucleus in a broad cone (of opening angle approximate to 120 degrees) inclined such that it intercepts the disk at grazing incidence. The cone opening angle implied by this model is considerably broader than would be expected in the simplest unified schemes for broad and narrow line AGN. It is also difficult to reconcile with the morphology of the nuclear narrow line region as revealed by Hubble Space Telescope images, if this is due solely to anisotropic illumination by the radiation field. However, it is more likely that the configuration of the emitting gas in the nucleus is the product of interactions with the radio ejecta.