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 photoion izing shocks in nearby objects. Photoionizing shocks are observed in H erbig-Haro objects and supernova remnants (SNRs), where there are no s trong, external sources of ionizing radiation, removing the ambiguitie s that persist in the regions surrounding active galactic nuclei. We s how that in one SNR, the radiation field produced behind fast shocks i s found to photoionize the precursor gas and generate a Seyfert-like s pectrum. We conclude that photoionizing shocks are a viable mechanism for generating the emission-line spectra observed in the narrow line r egions in active galaxies, but only under those circumstances where sh ocks are likely to be formed, e.g., through the interaction of a radio jet with the ISM. In those objects where sharp, straight-edged ioniza tion cones are observed, beamed ionizing radiation from the active nuc leus is preferred over photoionizing shocks. We show, however, that ph otoionizing shocks may help to resolve the UV photon deficit problem i n some objects because a significant fraction of the ionizing radiatio n is emitted as line emission in the EUV. In fact, the UV continuum fl ux near 1300 Angstrom from a 400 km s(-1) shock is similar to 100 time s lower than that from a power law containing the same number of ioniz ing photons. We discuss the implications of the strengths of collision ally excited UV lines, the weakness of Ca II lines, and the high elect ron temperatures derived from the [0 III] (lambda 4959+lambda 5007)/la mbda 4363 ratio for the ionization mechanism. The observed extended so ft X-ray emission in nearby AGNs may originate in shock-heated gas.