ippenko 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 is a compact non-thermal radio source, Sgr A*, at the center, with loops and filaments of non-thermal radio emission apparently emerging from it. Far infrared and radio-frequency velocity measurements suggest but do not prove the presence of a black hole with mass 5 x 106 M. There may also be a small, highly variable x-ray source at the nucleus. However, its x-ray luminosity, about 2 x 1038 erg s-1 is smaller by a factor of approximately 100 than that of the nucleus of M 81. This is probably the best estimate or upper limit to the activity of a possible AGN in our Galaxy (Genzel and Townes 1987, Fabbiano 1988).