Ultraviolet Imaging and Spectroscopy of LINERs

I review the UV properties of LINERs (low-ionization nuclear emission-line regions), based mostly on the recent {\it HST} UV imaging survey of nearby galaxies by Maoz et al. (1995). 25 of the galaxies in the northern subsample host a LINER nucleus. Six of these display a prominent compact ($<$ few pc) nuclear UV ($\sim 2300$\AA) source in the {\it HST} images. The remaining 19 LINERs are ``UV-dark'', with no detectable compact nuclear source. In the six UV-bright objects, the UV flux is correlated with H$\alpha$ flux. When extrapolated beyond the Lyman limit, the UV luminosity is sufficient to produce the observed H$\alpha$ luminosity through photoionization. Some LINERs therefore have a UV continuum source consistent with the expectations from the micro-quasar hypothesis. The 19 UV-dark objects are comparable in H$\alpha$ flux and luminosity to the UV-bright objects, and their darkness is not a detection-limit problem. Extinction by the host galaxy disk or other foreground dust cannot explain the entire effect. I consider several hypotheses explaining why only $\sim 25\%$ of LINERs display a central UV source: Obscuration of the UV source by NLR dust or a molecular torus; the UV source is ``turned-off'' most of the time; photoionization by an extended population of old stars; or, most LINERs are not photoionized objects. I discuss observational tests that will soon discriminate between these possibilities.