2.1. Seyfert 1 and Seyfert 2 galaxies
2.1.1. The emission line spectrum of Seyfert 2 galaxies
Seyfert 2s are characterized by a spectrum having both strong high- and low- ionization emission lines. The emission line spectra of Seyfert 2s are not accurately described by simple photoionization models of single clouds.
To explain both high and low ionization lines, the narrow-line region (NLR) must be composed of a mixture of dust-free, metal depleted (~ 0.5 solar, except for nitrogen which may be enhanced relative to the solar value by a factor 2) clouds with a radius-independent range in densities (102 to 105 cm-3) distributed over a range of distances from the nucleus. To encompass the observed range of line intensities relative to H, it is necessary to vary the spectral energy distribution incident on the clouds by adding a varying contribution of a hot blackbody (T ~ 2-6 105 K) to a steep X-ray power-law spanning 13.6 to 100 keV (; ).
Véron & Véron-Cetty (1986) found that the width of the narrow emission lines correlates with the Hubble types in Seyferts, earlier types having broader lines; they suggested that these line widths could correlate with the mass of the bulges. This was confirmed by  who found a correlation between line width and nuclear stellar velocity dispersion suggesting that gravitational motion plays an important role in the narrow line velocity field.
The redshifts obtained from the nuclear emission lines in Seyferts are on average smaller than the systemic velocities by about 60 km s-1, with differences reaching 250 km s-1 ().