Optical Spectroscopy of LINERs and Low-Luminosity Seyfert Nuclei

An unprecedentedly large number of LINERs has been discovered in a recently completed optical spectroscopic survey of nearby galaxies, allowing several statistical properties of the host galaxies and of the line-emitting regions to be examined reliably for the first time. As a consequence of the many detections and some revised classifications, the detailed demographics of emission-line nuclei have been updated from those given in older surveys. Consistent with previous studies, it is found that LINERs are extremely common in the present epoch, comprising approximately 1/3 of all galaxies with BT 12.5 mag. If all LINERs are nonstellar in origin, then they are the dominant constituents of the active galactic nucleus population. Many fundamental characteristics of LINERs closely resemble those of lowluminosity Seyfert nuclei, although several aspects of their narrow-line regions appear to differ in a systematic manner. These differences could hold important clues to the key parameters controlling the ionization level in active nuclei. Lastly, a substantial fraction of LINERs has been found to contain a broad-line region, yielding direct evidence, at least in these objects, of a physical link between LINERs and classical Seyfert 1 nuclei and QSOs.