The narrow-line region in the Seyfert 2 galaxy NGC 3393

The narrow-line region (NLR) of the Seyfert 2 galaxy NGC 3393 is dominated by a symmetric structure which appears as S-shaped arms in Hubble Space Tel escope (HST) images. These arms, which occupy the central few arcseconds of the nucleus, border a linear, triple-lobed radio source. We use HST imagin g and spectra, ground-based optical images, long-slit spectra, Fabry-Perot imaging spectroscopy, and VLA radio data to perform a detailed investigatio n of the kinematics and ionization of the line-emitting gas in NGC 3393 and of its relationship with the relativistic gas responsible for the radio em ission. The excitation map [O III] lambda 5007(H alpha + [N II] lambda lamb da 6548, 6584) shows a biconical structure, consistent with the anisotropic nuclear ionizing radiation expected in the unified scheme. Extrapolation t o ionizing frequencies of our upper limit to the 2100 Angstrom flux of the nuclear source provides a factor greater than or equal to3 x 10(4) too few ionizing photons to account for the recombination line emission, which also suggests that the nuclear ionizing source radiates anisotropically. Howeve r, the kinetic energy of the outflow is sufficient to power the line emissi on via photoionizing shocks, and a tentative detection of extended UV emiss ion is consistent with this model. Furthermore, the broad component of the emission lines has a similar orientation and spatial extent as the triple r adio source. Nevertheless, other tests are inconsistent with the photoioniz ing shock model-there is no correlation between local velocity dispersion, surface brightness, and excitation, and the gaseous abundances of [Ca II], Al II], and Mg II are much lower than expected if these species have been l iberated into the gas phase through grain destruction by shocks. We conclud e that the radio lobes appear to have created denser regions of gas on thei r leading edges, thus forming the S-shaped arms, but that the ionization is most likely due to photoionization by an obscured central source.