Low-ionization active galactic nuclei - X-ray or shock heated?

Some difficulties were experienced in connection with early photoionization models for the narrow-line regions of Seyfert galaxies. These difficulties could be overcome by the inclusion of charge transfer reactions. In connection with the present investigation, new photoionization models have been calculated for the forbidden-line regions, taking into account recent theoretical and observational developments. It is found that the entire range of excitation conditions observed in quasars, Seyfert galaxies, and 'Liners' (low-ionization nuclear emitting regions) can be reproduced by varying a single parameter which is related to the photon flux at the face of a cloud. There is some evidence that obscuration by the broad-line clouds may be responsible for the reduced flux seen by the narrow-line clouds in Liners and some low-luminosity Seyfert galaxies. It is shown that Einstein X-ray luminosities of several Liners and transition objects overlap with the weakest Seyfert galaxies. The implications of these findings for the classification of active galactic nuclei are discussed.