Hubble Space Telescope/Faint Object Spectrograph spectroscopy of spatially resolved narrow-line regions in the Seyfert 2 galaxies NGC 2110 and NGC 5929

We present the results of UV and optical Hubble Space Telescope/Faint Object Spectrograph spectroscopy of bright, extranuclear regions of line emission in the Seyfert galaxies NGC 2110 and NGC 5929. We have obtained spectra of the brightest region of the "nuclear jet" of NGC 2110 (75 pc from the nucleus) and of the southwest emission-line cloud of NGC 5929 (90 pc from the nucleus), in the G130H (1090-1605 Angstrom), G190H (1570-2310 Angstrom), G400H (3235-4780 Angstrom), and G570H (4570-6820 Angstrom) configurations. The observed line ratios are compared with the predictions of the two component (matter- and ionization-bounded, MB-IB), central source photoionization models of Binette, Wilson, & Storchi-Bergmann and of the fast, photoionizing ("'autoionizing"') shock models of Dopita & Sutherland. In both objects, the significant reddening inferred from the Balmer line ratios and/or its uncertainty limit the utility of the ultraviolet carbon lines C IV lambda 1549 and C III] lambda 1909 for discrimination between the central source and shock-induced photoionization mechanisms. In NGC 2110, shock + precursor models with a shock velocity of similar or equal to 400 km s(-1) provide a better match to the data than the MB-IB models. However, given the simplifying assumptions made in the latter models, photoionization by a central source cannot be ruled out. We investigate whether photoionizing shocks in the emission-line region of NGC 2110 can power the extended, soft X-ray emission north of the nucleus and find that shock velocities higher than 500 km s(-1) are required. In NGC 5929, the MB-IB models have problems reproducing the strengths of the neon lines, while shock + precursor models with a velocity similar or equal to 300 km s(-1) provide a good match to the data. For both galaxies, the emission-line powers and volumes of the ionized gas inferred from observations imply that both the preshock density (n(0)) and magnetic parameter (B-0/n(0)(1/2)) must be relatively high (n(0) > 10 cm(-3); B-0/n1/20 similar or equal to 4 mu G cm(3/2)) for the photoionizing shock models to be viable.