The spectral classification of Seyfert galaxies into two types,
Seyfert 1, with broader HI, He I and He II emission lines, and
narrower forbidden lines, and Seyfert 2, with H I, He I and He II
emission with essentially the same widths as the forbidden lines, both
comparable with the widths of the forbidden lines in Seyfert 1
galaxies, is well known from the papers of
Khachikian and Weedman
(1974).
We later introduced the additional intermediate type Seyfert
1.5, meaning an object whose H I emission lines have composite
profiles, with both broad and narrow components easily recognizable
(Osterbrock and Koski
1976;
Osterbrock 1977).
Still later I defined
two further types, Seyfert 1.8 and 1.9, in which in addition to the
narrow components, very weak but recognizable broad components of
H
and H
, or of
H alone, respectively, are present
(Osterbrock 1981).
These additional types have been used by some but not all
researchers.
Classification ideally is a strictly observational process, independent of theory, that different people can do (with good data), and get the same answers. The types may be defined by standards, as in the MK system of stellar spectral classification, or by definitions, as in the Khachikian and Weedman system of Seyfert galaxy spectral classification. These definitions however, have been modified in the course of time; for instance "narrow" line widths of all the emission lines (FWHM < 300 km s-1) is no longer considered a criterion to eliminate a candidate galaxy from the Seyfert 2 class. The reason is that many galaxies have been discovered with this property whose emission-line spectra are otherwise indistinguishable from the classically known Seyfert 2's (Veron 1981; Phillips, Charles and Baldwin 1983).