Abstract
Low-ionization nuclear emission-line regions (LINERS), defined by the criteria [O II] λ3727/[O III] λ5007 => 1 and [O I] λ6300/[O III] λ5007 => 1/3, are generally thought to be related to classical active galactic nuclei powered by nonstellar continua. Based on the results of new photoionization models, however, we suggest that those genuine LINERs having weak [O I] λ6300 emission (i.e., [O I]/Hα <~ 1/6) may instead be powered by hot (T_eff_ ~> 45,000 K) main-sequence O stars irradiating solar-metallicity clouds of gas. The derived ionization parameter (U) is in the range 10^-3.7^ to 10^-3.3^, a factor of ~20 lower than in normal H II regions. Very high [N II]/Hα ratios sometimes observed in LINERs may require a selective enhancement of nitrogen above the solar value. The presence of unusually hot stars (spectral type O3-O4) in such high-metallicity environments, together with low values of U, may be a natural consequence of the high gas densities, pressures, and masses in galactic nuclei. We also find that many weak-[O I] "LINERs" in the literature are not genuine LINERs, since they do not satisfy the [O I]/[O III] criterion; they were classified as LINERs only on the basis of their [O II]/[O III] or [N II] λ6583/Hα intensity ratios. These objects can easily be explained with our O-star photoionization models by increasing U up to 10^-3^.
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