A REEVALUATION OF THE EXCITATION MECHANISM OF LINERS

We present high-quality spectra of a sample of 13 low-ionization nuclear emission-line regions (LINERs) observed in the range approximately 3400-9800 angstrom at approximately 5-8 angstrom resolution. Starlight removal is achieved by the subtraction of a suitable absorption-line ''template'' galaxy, allowing accurate measurement of emission lines. We use the line fluxes to examine several candidates for the excitation mechanism of those LINERs whose emission-line gas is largely confined to radii less than or similar to 200 pc. We find that photoionization by a nonstellar ionizing continuum provides the best overall explanation for the spectra over the optical-to-near-infrared region. This is supported by the detection of broad Halpha emission, a correlation between line width and critical density of the forbidden lines, and pointlike X-ray emission in several of these objects. Wolf-Rayet stars or unusually hot O stars are less probable photoionization sources, especially for LINERs which exhibit broad Halpha emission. The prevalence of shock heating in galactic nuclei cannot be readily evaluated with the present plane-parallel shock models, although it is likely to be an important,excitation mechanism for LINERs with spatially extended emission. A small fraction of objects previously classified as LINERs may be undergoing active star formation in or near their nuclei.