DETECTION OF COMPACT ULTRAVIOLET NUCLEAR-EMISSION IN LINER GALAXIES

Low-ionization nuclear emission-line regions (LINERs), which exist in a large fraction of galaxies, may be the least luminous manifestation of quasar activity. As such, they may make possible the study of the A GN phenomenon in the nearest galaxies. The nature of LINERs has, howev er, remained controversial because an AGN-like nonstellar continuum so urce has not been directly observed in them. We report the detection o f bright (greater than or similar to 22 x 10(-16) ergs s(-1) cm(-2) An gstrom(-1)), unresolved (FWHM less than or similar to 0.1'') point sou rces of UV (similar to 2300 Angstrom) emission in the nuclei of nine n earby galaxies. The galaxies were imaged using the Faint Object Camera on the Hubble Space Telescope (HST), and seven of them are from a com plete sample of 110 nearby galaxies that was observed with HST. Ground -based optical spectroscopy reveals that five of the nuclei are LINERs , three are starburst nuclei, and one is a Seyfert nucleus. The observ ed UV flux in each of the five LINERs implies an ionizing flux that is sufficient to account to the observed emission lines through photoion ization. The detection of a strong UV continuum in the LINERs argues a gainst shock excitation as the source of the observed emission lines, and supports the idea that photoionization excites the lines in at lea st some objects of this class. We have analyzed ground-based spectra f or most of the northern-hemisphere galaxies in the HST sample and find that 26 of them are LINERs, among which only the above five LINERs ha ve a detected nuclear UV source. There are no obvious differences in t he optical line intensity ratios between the UV-bright and UV-dark LIN ERs. If all LINERs are photoionized, then the continuum source is unob scured along our line of sight in 5/26 approximate to 20% of LINERs. A lternatively, it can be argued that spectrally similar LINERs are prod uced by various excitation mechanisms, and that photoionization is res ponsible in only similar to 20% of the cases. The high angular resolut ion allows us to set upper limits, typically several parsecs, on the p hysical size of the compact star cluster or AGN-type continuum source that is emitting the UV light in these objects.