SPECTRAL SIGNATURES OF FAST SHOCKS .2. OPTICAL DIAGNOSTIC DIAGRAMS

In the first paper in this series, we presented a low-density grid of models of high-velocity photoionizing radiative shocks, including magn etic pressure support in the photoionization/recombination zone. Here we apply these models to the line ratios observed in narrow-line emiss ion regions in active galaxies. From a set of line diagnostic diagrams , we find that LINER galaxies, narrow-line radio galaxies, and cooling flow emission regions can be modeled in terms of fast shocks in a rel atively gas poor environment. Emission from a photoionized precursor o f the shock is either weak or absent. On the other hand, the narrow-li ne regions associated with Seyfert 2 and 1.5 galaxies and the energeti c luminous IR galaxies can be understood as fast shocks in a gas-rich environment, in which the EUV photons produced in the shock are fully, or mostly, absorbed in the shock precursor H II region. For LINER-lik e objects, shock velocities required range from 150 to 500 km s(-1), b ut the Seyfert spectra require shocks typically in the range 300-500 k m s(-1). These figures are comparable to the observed line widths in b oth these classes of object. The magnetic parameter that characterizes these shocks is about 2 < B/n(1/2) < 4 mu G cm(3/2), typical of the g eneral interstellar medium. Our fast shock models are capable of expla ining the long-standing ''temperature problem'' of active galactic nuc lei, in which the electron temperatures observed are found to be syste matically higher than predicted by photoionization models. For Seyfert galaxies, the preshock densities are clearly higher than the 1 cm(-3) used in the model grid. Finally, we find evidence that nitrogen is en hanced above solar values in both Seyfert and in LINER nuclei.