LINERs and their Physical Mechanisms

I review the basic properties of low-ionization nuclear emission-line regions (LINERs), as well as the main ionization mechanisms thus far proposed for them. There is substantial heterogeneity among LINERs, especially those with extended kpc-scale emission. An extensive survey by Ho, Filippenko, & Sargent quantifies with unprecedented accuracy the optical emission-line intensity ratios in the nuclei of a complete sample of 486 nearby, bright galaxies. Over 40% of all early-type galaxies in the sample can be classified as LINERs or LINER/H II transition objects. A nonstellar continuum is detected in many LINERs, suggesting that they are genuine AGNs powered by accretion onto a compact object. Broad H alpha emission similar to that of Seyfert 1 nuclei is seen in many LINERs. In some LINERs, the broad H alpha emission has two well-separated peaks, probably indicative of an accretion disk.