Published in "Active Galactic Nuclei", 1988, eds. H. R. Miller and P. J. Wiita


Donald E. Osterbrock

Lick Observatory
Board of Studies in Astronomy and Astrophysics
University of California Santa Cruz, California 95064

Abstract. Seyfert galaxy emission-line spectra provide clues to the structure of active galactic nuclei and the nature of their central sources. Classification of their spectra organizes a wide range of properties; they do not all fit into a single one-parameter sequence. The luminosity functions of the individual types provide constraints on their evolution and on possible geometries, but are difficult to determine because objective-prism and color surveys tend to be most efficient in finding blue objects. The IRAS survey has identified many more Seyfert 2 galaxies, which tend on the average to be more heavily reddened than the previously known objects. The discovery of very faint broad Hα emission components in many emission-line galaxies by Filippenko and Sargent indicates the continuity of the general AGN process to very low luminosity levels. The narrow-line profiles of all types of Seyfert galaxies have the same general form, indicating a single type of velocity field. It is evidently mostly radial, but not necessarily spherical. There are differences in detail, which on the average tend to be correlated with the spectral type, but these are by no means invariably followed at the level of individual galaxies. The broad-line profiles show a great range in properties. Combinations of radial and rotational velocity fields give the best fits with the observed profiles, but there are theoretical difficulties with this picture, particularly with rotational flow.

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