6 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, Filippenko 1989). Presumably there are `mini-AGNs' with smaller masses and mass accretion rates than Seyfert 1s and QSOs. Probably the values previously cited for the nucleus of M 81 are roughly applicable. There are probably wide ranges in both mass and mass accretion rate, for the luminosity () and FW0I (determined by M and R) are not well correlated and the objects do not form a one-parameter sequence in these observational parameters (Keel 1983a).
The lowest-luminosity AGN currently known is the nucleus of the late-type, nearby, dwarf spiral NGC 4395 (Filippenko and Sargent 1989). Its absolute magnitude is only MB -10 (in the continuum). Its narrow emission lines have FWHMs < 60 km s-1, but are otherwise characteristic of a Seyfert 2 nucleus, with [O III] 5007 / H 7, strong [O I] 6300 and [S II] 6716, 6731, and measurable He II 4686, [Ne V] 3426 and [Fe X] 6375. In addition H and H have weak broad components with FW0I ~ 4000 and 7000 km s-1 respectively. Thus this nucleus fits in most ways the definition of a very low-luminosity Seyfert 1.8 nucleus with unusually narrow lines. The luminosity in the broad H emission component is L(H) 1.2 x 1038 erg s-1, down by about a factor 10 from M 81. This broad component would probably not be detectable in an earlier-type spiral galaxy with a correspondingly more luminous stellar nucleus.
Note however that all LINER-type spectra are not the results of photoionization. Morphologically, a certain number of LINERs are extended, nebulous objects in which the form strongly suggests that shock-wave heating is in progress. None of these show He II 4686. Clearly LINERs include objects of quite different physical nature; but those in the nuclei of galaxies are AGNs photoionized by a hard but weak spectrum (Heckman 1987, Filippenko 1989).
Finally we may briefly discuss the question as to whether our Galaxy has an AGN at its nucleus. The optical extinction to the center (AV 35 magnitudes) is so high that there is no chance to observe the nucleus directly in the optical region. There i