6.1. QSOs and Seyfert 1 galaxies
Seyfert galaxies were first recognized as galaxies, then as galaxies with unusually luminous nuclei with strong, broad emission lines. Quasars were first identified as quasi-stellar radio sources with similar strong, broad emission lines. All the evidence since then has tended to indicate these are but two somewhat distance-dependent names for similar physical entities. Many of the most luminous radio galaxies are morphologically classified as N galaxies, meaning bright, near-quasi-stellar objects, with only a faint, barely detectable extended component or `fuzz'. Many objects such as I Zw 1, II Zw 1, Mrk 876, and 3C 120 have appeared both as Seyfert galaxies and QSOs in various catalogues. This continuity of physical appearance has been particularly emphasized by Morgan and Dreiser (1983). In recent years, using high-quality digital detectors and subtraction techniques, various authors have been able to detect faint galaxy images about the AGNs that had previously been described as quasars or QSOs. Out to about z 0.5, nearly every such object has yielded to these techniques. The emission-line spectra show no consistent differences. Furthermore, high-quality spectra show an underlying integrated stellar absorption-line spectrum in many objects, in addition to the strong featureless continuum of the central source. In many cases these spectra are earlier in spectral type than normal spirals, indicating the results of recent star formation in the active galaxies (see also section 7.5). Two very good reviews of this material, with many references to the original papers, are by Hutchings (1983) and Miller (1985).
As mentioned in section 2.4, the observed ratio of H luminosity to continuum luminosity, L(H) / LC is constant over a wide range of luminosity from Seyfert 1 galaxies through QSOs, suggesting that the form of the spectrum and the amount of the luminosity absorbed (covering factor / 4) is also constant (Yee 1980, Shuder 1981). Attempts have been made to measure the covering factor ( / 4 subtended at the source by the photoionized clouds) from the observed ratio of soft x-ray flux (which is absorbed in the photoionization processes) to the hard x-ray flux (which may pass through unabsorbed if the column density is not too large). These suggest that the covering factors are larger for low-luminosity objects than for higher-luminosity ones (Lawrence and Elvis 1982, Reichert et al 1985). However, more recent, higher-resolution measurements of more objects show that the x-ray situation is quite complicated. Nevertheless they tend to support this decrease in covering factor with luminosity, but with considerable scatter (Mushotzky 1988, Turner and Pounds 1989). Similarly, comparison of the equivalent widths of both L and C IV 1549 in high-luminosity QSOs with those in Seyfert 1 galaxies, show a decrease (by about a factor two) in covering factor over this range of luminosity (Wu et al 1983). Again, the scatter is large (Wilkes 1986).
6.2. Quasars and radio galaxies
Quasars and broad-line radio galaxies appear to be related much as QSOs and Seyfert 1 galaxies are. The broad-line radio galaxies are almost invariably N, cD, D or E galaxies rather than spirals, and are rare among the E galaxies. There are few if any published surveys of D, cD or N galaxies that are not radio galaxies, but practically no radio-quiet Seyfert galaxies of these morphological types are known. Most probably the radio plasma that escapes from the nucleus along the jets in relatively gas-free cD, D and E galaxies is stopped by interaction with interstellar gas in Seyfert galaxies.
The outstanding spectral difference between the radio-quiet and radio-loud objects is that many Seyfert 1 galaxies and QSOs have strong, broad Fe II emission features in the optical region, while few quasars and broad-line radio galaxies do. This subject has been reviewed by Osterbrock (1985), who gives many references to the original papers. The ultraviolet Fe II multiplets arising from the same upper levels as the observed optical features 