Outflow densities and ionization mechanisms in the NLRs of the prototypical Seyfert galaxies NGC 1068 and NGC 4151

Despite being thought to play an important role in galaxy evolution, the true impact of outflows driven by active galactic nuclei (AGNs) on their host galaxies is unclear. In part, this may be because electron densities of outflowing gas are often underestimated: recent studies that use alternative diagnostics have measured much higher densities than those from commonly used techniques and consequently find modest outflow masses and kinetic powers. Furthermore, outflow ionization mechanisms – which are often used to probe acceleration mechanisms – are also uncertain. To address these issues, we have analysed archival HST/STIS spectra of the inner regions (r < 160 pc) of the nearby prototypical Seyfert galaxies NGC 1068 and NGC 4151, which show evidence of warm-ionized outflows driven by the central AGN. We derive high electron densities (103.6 < ne < 104.8 cm−3) using the transauroral [O II] and [S II] emission line ratios for the first time with spatially resolved observations. Moreover, we find evidence that the gas along the radio axis in NGC 1068 has a significant AGN-photoionized matter-bounded component, and there is evidence for shock-ionization and/or radiation-bounded AGN-photoionization along the radio axis in NGC 4151. We also note that the outflow extents are similar to those of the radio structures, consistent with acceleration by jet-induced shocks. Taken together, our investigation demonstrates the diversity of physical and ionization conditions in the narrow-line regions of Seyfert galaxies, and hence reinforces the need for robust diagnostics of outflowing gas densities and ionization mechanisms.