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A.J. Barth (Harvard-Smithsonian CfA), L.C. Ho (Carnegie Observatories), A.V. Filippenko (U.C. Berkeley), H.-W. Rix (MPIA Heidelberg), W.L.W. Sargent (Caltech)
Ground-based optical studies of low-luminosity AGNs face the problem that the nuclear continuum light observed from the ground is dominated by starlight, making it difficult to detect nonstellar continua and broad lines. Small-aperture spectra taken with the STIS spectrograph on HST can largely circumvent this problem. We have obtained STIS optical spectra covering 6300-6860 A of the nuclei of six low- to moderate-luminosity, broad-lined AGNs of Seyfert and LINER type. In NGC 4579, the spectra reveal a remarkable broad H\alpha emission line with a full-width near zero intensity of 18,000 km/s, which may originate from an accretion disk. This very broad line may be a transient feature, but it could have gone undetected in previous ground-based spectra because of its low amplitude relative to the continuum.
The spectra also reveal new structure in the narrow-line regions. In the LINERs NGC 4579 and NGC 1052, the ratio of [S II] 6717,6731 shows a pronounced gradient indicating a steep rise in density toward the nucleus. The ratio [O I] 6300 / [S II] 6717, 6731 indicates a likely nebular density of \gtrsim105 cm-3 at the nuclei. Such high-density clouds are consistent with expectations from previous observations of linewidth-critical density correlations in AGNs. The direct detection of high-density clouds in these LINERs supports photoionization as the dominant ionization mechanism in the narrow-line gas.