inosity, 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 4686 emission. It can only be understood as resulting from photoionization by a hard spectrum. The models calculated on this basis fit LINERs well (Ferland and Netzer 1983, Halpern and Steiner 1983, Keel 1983a, Stasinska 1984a, Binette 1985).
Very strong confirming evidence of photoionization is available from IUE observations of one LINER, NGC 4579. They show that there is a point-like ultraviolet source at the nucleus of this galaxy, with an observed flux which, with assumed power-law index n = 1.4, fits the measured x-ray flux and provides the required number of ionizing photons to power the observed H flux (Goodrich and Keel 1986).
Many LINERs have been detected in spiral galaxies. For instance in a well defined sample, observed and with the continuum carefully subtracted to reveal the presence of very weak nuclear emission lines, all showed H and [N II]. Of these nuclei 5% were Seyferts, and of the remainder approximately 80% of the Sa and Sb nuclei showed LINER-type spectra. This percentage dropped steeply through Sc, to essentially zero at Scd, probably at least in part because of the increased strength of the H II region type spectra in the nuclei of these objects, resulting from recent star formation. Furthermore, approximately 10% of these LINERs in this survey show weak, broad H lines (FW0I 3000 km s-1). They might be classified as Seyfert 1.99 (a very weak, broad component of H barely detectable!) on the traditional scheme. They must be an extension of `typical' (previously known) AGNs to smaller black-hole masses and lower accretion rates (Stauffer 1982, Keel 1983b).
In addition to the spirals, the nuclei of a few elliptical galaxies have LINER spectra. Two bright, well studied examples are NGC 1052 and NGC 4278. In NGC 1052 a weak, broad H emission component has been detected, again indicating an AGN similar in general type to those in Seyfert 1 galaxies (Filippenko and Sargent 1985). Many more similar cases have been found by careful analysis of high signal-to-noise-ratio spectra (Filippenko 1985, Fil