LINERs as low-luminosity active galactic nuclei

Many nearby galaxies contain optical signatures of nuclear activity in the form of LINER nuclei. LINERs may be the weakest and most common manifestation of the quasar phenomenon. The physical origin of this class of objects, however, has been ambiguous. I draw upon a number of recent observations to argue that a significant fraction of LINERs are low-luminosity active galactic nuclei. The evidence in favor of this interpretation includes the similarity in the properties of the host galaxies of LINERs and Seyferts, the detection of broad-line regions, the detection of black holes in a handful of nearby galaxies that are spectroscopically recognized as LINERs, the presence of a compact nuclear source seen in the radio, ultraviolet, and X-rays, and the detection of nonthermal spectra in the hard X-ray band. The spectral energy distributions of LINERs differ dramatically from those of luminous AGNs, most notably in the absence of the "big blue bump." This and other characteristics suggest that the central engine in LINERs is fed by a very low accretion rate. (C) 1999 COSPAR. Published by Elsevier Science Ltd.