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Heckman (1980) has defined Liners as galaxies which have a nuclear optical spectrum dominated by emission lines from low excitation species (lambda3727 > lambda5007 and lambda6300 > 1/3 lambda5007). Liners occur preferentially in galaxies of early Hubble type. They are characterized by lines whose widths are similar to those in the narrow-line region of Seyferts but whose luminosity is generally low.

The excitation mechanism of Liners has been highly controversial. Excitation mechanisms that have been proposed include photoionization by a dilute power-law continuum, shock heating, cooling flows, and photoionization by very hot Wolf-Rayet stars or normal O stars ([123]). Liners may be due to a variety of apparently similar but physically distinct phenomena ([187]).