IUE SPECTRA AND PHOTOIONIZATION MODELS OF THE SEYFERT-2 GALAXIES NGC-7674 AND I-ZW-92

The physical conditions in the narrow-line regions of two Seyfert 2 ga laxies, NGC 7674 and I Zw 92, are examined using IUE spectra, publishe d optical spectra and multifrequency observations, and photoionization models. For each Seyfert galaxy, the emission-line fluxes were deredd ened using the He II lambda 1640/lambda 4686 ratio. Photoionization mo dels were calculated using a power-law index determined from the He II lambda 4686/H beta ratio; the index is very similar to that obtained from a fit to the observed multifrequency continuum from the infrared to the X-rays. The models were calculated in a way that minimized the number of assumptions, and given the uncertainties in the reddening co rrections, the calculated ratios match nearly all of the dereddened ra tios successfully. A multicomponent model (three components with diffe rent densities and ionization parameters) was required to fit the spec trum of I Zw 92, whereas a single component was sufficient for NGC 767 4. The CNO abundances are close to solar, although a reduced abundance of up to one-third solar for one or more of the heavy elements is pos sible. In contrast to a previous study of Mrk 3, dust inside the NLR c louds was not required to fit the spectra of these two Seyfert galaxie s, although the emission lines experience considerable reddening from external dust. Higher signal-to-noise spectra in the UV are essential for placing further restrictions on the reddening and physical conditi ons in the narrow-line regions of Seyfert galaxies.