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 Å), G190H (1570-2310 Å), G400H (3235-4780 Å), and G570H (4570-6820 Å) 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, {\amp} Storchi-Bergmann and of the fast, photoionizing (``autoionizing'') shock models of Dopita {\amp} 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 λ1549 and C III] λ1909 for discrimination between the central source and shock-induced photoionization mechanisms. In NGC 2110, shock+precursor models with a shock velocity of ~=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 ~=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 (n0) and magnetic parameter (B0/n1/20) must be relatively high (n0>10 cm-3 B0/n1/20~=4 μG cm3/2) for the photoionizing shock models to be viable. Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

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Published in The Astrophysical Journal, v. 523, no. 1, p. 147-162.

© 1999. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

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