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 ∼3400-9800 Å at ∼5-8 Å 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 ≲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 Hα 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 Hα 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.