If LINERs are powered by a nonstellar source, then LINERs clearly would be the most common type of AGNs known in the nearby regions of the universe. However, ever since their discovery, the physical origin of LINERs has been hotly debated. The recognition and definition of LINERs is based on their spectroscopic properties at optical wavelengths. In addition to the AGN scenario, the optical spectra of LINERs unfortunately can be interpreted in several other ways that do not require an exotic energy source (e.g., shocks, hot stars; see Ho et al. 1993 and Filippenko 1996 for reviews). As a consequence, it has often been suggested that LINERs may be a mixed-bag, heterogeneous collection of objects. While the nonstellar nature of some well-studied LINERs is incontrovertible (e.g., M81, M87, M104), the AGN content in the majority of LINERs remains unknown. Determining the physical origin of LINERs is more than of mere phenomenological interest. Because LINERs are so numerous, they have a tremendous impact on the specification of the faint end of the local AGN luminosity function, which itself bears on a range of issues. A number of recent developments provide considerable new insight into the origin of LINERs. I outline these below, and I use them to advance the proposition that most LINERs are truly AGNs (1)
1 Note that this
paper is concerned only with compact, nuclear LINERs (r
200 pc),
which are most relevant to the AGN issue. LINER-like spectra are
often also observed in extended nebulae such as those associated with
cooling flows, nuclear outflows, and circumnuclear disks.
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