The excitation mechanism of emission-line regions in Seyfert galaxies

We present Hubble Space Telescope UV/optical spectrophotometry and ground-based optical spectrophotometry of the brightest knots in the emission-line regions of the Seyfert galaxies NGC 5643 and NGC 5728. These objects have ionization cones, radio and soft X-ray emission, and evidence of nuclear outflows and winds, suggesting that shocks may be associated with the emission-line gas. Comparison of the UV/optical data with grids of ionization-bounded and mixed ionization-and matter-bounded photoionization models, and shock models with and without ionized precursors, are used to determine whether radiative shocks that photoionize the shock precursor material ("autoionizing shocks") are a significant excitation mechanism for the emission-line gas in these objects. Although the UV/optical spectra studied here are generally compatible with both photoionization and shock + precursor models, an unambiguous shock + precursor signature is not present. This suggests that mechanical energy input may not play the same dominant role in exciting these objects with weak radio jets as appears to be the case in strong radio jet sources such as M87.