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 luminosity, Elvis and Van Speybroeck (1982) predicted that it should be detectable as an x-ray source and did find it. Further x-ray data have shown that the nuclear source is relatively soft, with a power-law form spectrum -n with n > 2. This is steeper than the characteristic value n 0.7 for AGNs, but within the range of a few others. It can be very approximately fitted by an accretion-disk model corresponding to black-hole mass M 104-105 M and 10-4-10-3 M y-1, in which the x-ray emission arises in the disk itself (Fabbiano 1988). Long-slit spectra show narrow emission lines of [N I], [O III] and [Fe VII] (only a small range in wavelength was observed) in the NLR close to the nucleus, and give only an upper limit to the black-hole mass M 107 M (Keel 1989).
Many such `Low-Ionization Nuclear Emission-Line Regions' or LINERS have been identified. They are nuclei with emission-line spectra showing a generally lower-level of ionization than Seyfert 2s, but with the characteristic relatively strong [O I], [S II] and [N II] lines of an AGN. Many LINERs have relatively weak emission lines, and to detect them and to measure their strengths with any precision it is necessary to subtract the underlying galaxy absorption-line spectrum fairly accurately.
When the class of LINERs was first isolated by Heckman (1980), it was suggested that they might be objects in which shock-wave heating is the main energy-input mechanism, rather than photoionization by a hard spectrum. At relatively low levels of ionization (below log [O III] / H = + 0.5) the diagnostic ratios of figures 2, 3 and 4 do not distinguish between these two possibilities. However, it now seems much more likely that LINERs are simply the extension of Seyfert 2 nuclei to lower luminosities, smaller ionization parameters, and somewhat larger exponents n in the representative power-law photoionizing spectrum. Figures 1, 2 and 3 extend smoothly down to LINERS, and several nuclei with ionization near the Seyfert 2-LINER `boundary' log [O III] / H + 0.5 show weak He II 468