On the viability of gaseous ionization in active galaxies by fast shocks

We discuss the possible importance of fast, photoionizing shocks as an ionization mechanism for the narrow and extended narrow line regions in active galaxies by referring to observational evidence for photoionizing shocks in nearby objects. Photoionizing shocks are observed in Herbig-Haro objects and supernova remnants (SNRs), where there are no strong, external sources of ionizing radiation, removing the ambiguities that persist in the regions surrounding active galactic nuclei. We show that in one SNR, the radiation field produced behind fast shocks is found to photoionize the precursor gas and generate a Seyfert-like spectrum. We conclude that photoionizing shocks are a viable mechanism for generating the emission-line spectra observed in the narrow line regions in active galaxies, but only under those circumstances where shocks are likely to be formed, e.g., through the interaction of a radio jet with the ISM. In those objects where sharp, straight-edged ionization cones are observed, beamed ionizing radiation from the active nucleus is preferred over photoionizing shocks. We show, however, that photoionizing shocks may help to resolve the UV photon deficit problem in some objects because a significant fraction of the ionizing radiation is emitted as line emission in the EUV. In fact, the UV continuum flux near 1300 Angstrom from a 400 km s(-1) shock is similar to 100 times lower than that from a power law containing the same number of ionizing photons. We discuss the implications of the strengths of collisionally excited UV lines, the weakness of Ca II lines, and the high electron temperatures derived from the [0 III] (lambda 4959+lambda 5007)/lambda 4363 ratio for the ionization mechanism. The observed extended soft X-ray emission in nearby AGNs may originate in shock-heated gas.