ctor ( / 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 are very strong in all Seyfert 1 galaxies (Veron-Cetty et al 1983). They are observed in the broad-line radio galaxies as well, indicating that Fe+ is present in their nuclei, but in lower abundance or less highly excited (Wills et al 1985). The physical interpretation is not known at present. Models which schematically take into account increased heating by relativistic electrons seem to reproduce some but not all of the observed effects: they predict weakening of the Fe II optical multiplets but do not give the observed H I relative line strengths (Ferland and Mushotzky 1984, Cesar et al 1985).
6.3. Few high-luminosity Seyfert 2s
Although Seyfert 1 nuclei and QSOs form a continuous sequence, there are no, or only very few, Seyfert 2 nuclei with luminosities as bright as MB = -23. Only one `Seyfert 2 QSO' has been reported, E 0449-184, originally discovered as an x-ray object. Its redshift is z = 0.338 and its absolute magnitude MB = -23.2 (assuming H0 = 50 km s-1 Mpc-1 as the value of the Hubble constant). There is no sign of any broad emission lines in its spectrum, but it has strong, typical Seyfert 2 narrow emission lines, with FWHM 600 km s-1 (Stocke et al 1982). This is larger than average for a Seyfert 2, but fits well on the correlation (with considerable scatter) between L([O III]) and FWHM.
The fact that high-luminosity Seyfert 2 AGNs are so rare must be incorporated into any overall model of AGNs. On the traditional picture it means that a high-luminosity, strong continuum from the central source, and an extensive BLR always go together. On the `hidden-BLR' picture, it means that the occulting torus becomes very weak or disappears around MB = -23 (Miller and Goodrich 1990).
Although QSOs and Seyfert galaxy nuclei make up most of the known AGNs, lower-luminosity ones certainly also exist. One is the nucleus of M 81, in which very weak, broad H emission was detected by Peimbert and Torres-Peimbert (1981) and confirmed by Shuder and Osterbrock (1981). Based on the correlation between broa H and x-ray lum