The Narrow-Line Regions of Seyfert and Radio Galaxies

Abstract

AbstractThis paper reviews the morphology, excitation, and ionization of the gas in the narrow and extended narrow line regions (NLRs and ENLRs) in AGNs. Many high-resolution images of the NLRs of Seyfert galaxies are now available from HST. In some galaxies, these images reveal straight, sharp-edged V-shaped structures (believed to be the projections of 3-dimensional cones), while in others there is a close association between the ionized gas and the radio components. The latter effect arises through radiative shocks driven into the surrounding interstellar gas by the outwardly moving radio components or jets. Photoionization models of ionization-bounded clouds are quite successful in reproducing the line intensity ratios, but a number of problems remain. These problems may be resolved if matter-bounded clouds are also present. Differences between the excitation levels of different NLRs and ENLRs may reflect variations in the fraction of matter-bounded clouds as well as variations in the ionization parameter. I discuss the issue of whether the radiation responsible for photoionizing the gas in Seyfert 2 galaxies originates from a hidden, compact type 1 Seyfert nucleus or from photoionizing shocks within the NLR. These two scenarios are hard to distinguish between. The photoionizing-shock model requires surprisingly high pre-shock densities, raising the question of whether the shocks are sufficiently energetic to power the observed line emission.