We study the spectroscopic characteristics of low ionization nuclear emission-line regions (Liners) in galaxies, and compare them with other active galactic nuclei. We show the results of new, extensive photoionization model calculations and use them to investigate line ratio diagrams, such as [O III] λ5007/Hβ versus [O II] λ3727/[O III] λ5007, and [O I] λ6300/[O III] λ5007 versus [O III] λ3727/[O III] λ5007. On the diagrams broad line objects, Seyfert 2 galaxies, and Liners form a continuous smooth sequence of decreasing ionization parameter. This suggests a common origin for line excitation in all of them. The observed optical continuum of Liners is also consistent with ionization by a power-law source. The intensity of several weak lines such as [O III] 4363, He I 5876, He II 4686, and the Balmer decrement can decide between photoionization and the other proposed excitation mechanism-viz., shock wave. We believe that photoionization is more plausible, and that Liners are, indeed, another group of active nuclei.

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Published in The Astrophysical Journal, v. 264, no. 1, p. 105-113.

© 1983. The American Astronomical Society. All rights reserved.

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