Spectral Signatures of Fast Shocks. II. Optical Diagnostic Diagrams

Abstract

In the first paper in this series, we presented a low-density grid of models of high-velocity photoionizing radiative shocks, including magnetic pressure support in the photoionization/recombination zone. Here we apply these models to the line ratios observed in narrow-line emission 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 relatively gas poor environment. Emission from a photoionized precursor of the shock is either weak or absent. On the other hand, the narrow-line regions associated with Seyfert 2 and 1.5 galaxies and the energetic 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-like objects, shock velocities required range from 150 to 500 km s-1, but the Seyfert spectra require shocks typically in the range 300-500 km s-1 These figures are comparable to the observed line widths in both these classes of object. The magnetic parameter that characterizes these shocks is about 2 < 4 μG cm3/2, typical of the general interstellar medium. Our fast shock models are capable of explaining the long-standing "temperature problem" of active galactic nuclei, in which the electron temperatures observed are found to be systematically 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 enhanced above solar values in both Seyfert and in LINER nuclei.