Evidence for a physically compact narrow-line region in the Seyfert 1 Galaxy NGC 5548

We have combined Hubble Space Telescope/Faint Object Spectrograph and ground-based spectra of the Seyfert 1 galaxy NGC 5548 to study the narrow emission lines over the 1200-10000 Angstrom region. All of the spectra were obtained when the broad emission line and continuum fluxes were at a historically low level, allowing us to determine accurately the contribution of the narrow-line region (NLR) to the emission lines. We have generated multicomponent photoionization models to investigate the relative strength of the high-ionization lines compared to those in Seyfert 2 galaxies and the weakness of the narrow Mg II lambda 2800 line. We present evidence for a high-ionization component of NLR gas that is very close to the nucleus (similar to 1 pc). This component must be optically thin to ionizing radiation at the Lyman edge (i.e., tau(o) approximate to 2.5) to avoid producing [O I] and Mg II in a partially ionized zone. The very high ionization lines (N V, [Ne V], [Fe VII], [Fe x]) are stronger than the predictions of our standard model, and we show that this may be due to supersolar abundances and/or a "blue bump" in the extreme ultraviolet (although recent observations do not support the latter). An outer component of NLR gas (at only similar to 70 pc from the continuum source) is needed to produce the low-ionization lines. We show that the outer component may contain dust, which further reduces the Mg II flux by depletion and by absorption of the resonance photons after multiple scatterings. We show that the majority of the emission in the NLR of NGC 5548 must arise within similar to 70 pc from the nucleus. Thus, the NLR in this Seyfert 1 galaxy is very physically compact, compared to the typical NLR in Seyfert 2 galaxies.