Future studies should refine the statistical completeness of the current AGN surveys. The Palomar survey, while a significant improvement compared to previous optical surveys, is nonetheless limited by ground-based seeing and by host galaxy contamination. In early-type galaxies, where the contamination from starlight is strong, there is a practical limit to which weak emission lines can be extracted from the total spectrum. On the other hand, in late-type galaxies the strong emission from H II regions in and near the nucleus can easily mask the fainter signature of a weak AGN that might be present. The reported low incidence of AGNs in late-type galaxies, therefore, may be misleading and needs to be verified. A significant increase in sensitivity to weak nuclear emission can be achieved with high-angular resolution spectroscopy with HST. A follow-up program to assess the completeness of the Palomar survey is being performed with the Space Telescope Imaging Spectrograph.
The recent availability of high-quality multiwavelength observations has provided important clues toward resolving the LINER mystery. As outlined in this review, while the data collectively, and in some instances even individually, do support the AGN interpretation for LINERs, it is still premature to draw quantitative, statistical conclusions concerning the AGN content in LINERs. The most outstanding unanswered question is the fraction of the LINER 2 population which are genuine AGNs. The cleanest test of the AGN hypothesis must rely on high-resolution observations in the hard X-ray band, which is the least affected by absorption and by confusion from young stars. Detection of a single compact hard X-ray source coincident with the nucleus would constitute strong evidence for the existence of an accretion-powered source. Such an experiment is being planned for the ACIS camera (resolution ~ 0".5) on AXAF.
My work is supported by NASA grants from the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555. This paper was written during a visit to the Academia Sinica's Institute of Astronomy and Astrophysics; I thank the members of the Institute, and especially its director, K. Y. Lo, for their warm hospitality.