There is little doubt, by now, that at least some members of the LINER class
truly do belong in the AGN family. LINERs turn out to share a surprisingly
large number of traits found in low-luminosity Seyfert nuclei. The global
characteristics of their host galaxies (Hubble type, presence of a bar,
inclination, and total luminosity), as seen at optical wavelengths at
least, are essentially indistinguishable. If the narrow
H
line can be
regarded as an approximate gauge of the power output of the central source, it
also appears that the luminosity of the nucleus is not a useful predictor of
the ionization level. Contrary to a popular misconception, not every weak
emission-line nucleus in an early-type galaxy is a LINER; there are plenty of
Seyfert nuclei with emission lines just as faint as, if not even fainter than,
those seen in LINERs. Judging by the frequency with which asymmetric
narrow-line profiles are observed in LINERs, as well as the clear preference
for the asymmetry to occur toward the blue half of the line center, the bulk
velocity field of the NLRs in LINERs must have a non-negligible radial
component, as has been known to be the case in Seyferts. Finally, of great
importance, the resemblance between LINERs and Seyferts has now been shown to
extend to the presence of a BLR, one of the definitive trademarks of the AGN
phenomenon: broad H emission is
detected in roughly 25% of LINERs.
A continuous, wide range of ionization levels clearly exists among AGNs, and it should be obvious that any clear-cut division of AGNs into ``high'' and ``low'' excitation flavors is arbitrary at some level. Instead, we should turn our attention to the more general question of what key parameters control the ionization level in AGNs. Whenever possible, I have attempted to broach this issue by referencing the observed properties of LINERs with those of Seyferts, which, for the purposes of this discussion, are implicitly assumed to be a well-understood class of objects. Rudimentary though these comparisons may be, some informative patterns have emerged.
As shown in Section 6, the NLRs in LINERs differ from those in Seyferts in several aspects. The line-emitting regions tend to have lower density (at least for the low-density component), lower internal reddening, but larger line widths. Could these indications be telling us something about differences in the structure of the NLR between the two types of objects? An additional clue, although it offers no simple explanation of the above-mentioned observables, is furnished by the apparently higher rate (by about a factor of two) in which density stratification, as identified through profile variations in lines with dissimilar critical densities, is seen in LINERs relative to Seyferts (see Section 6.4).
Whittle (1992a, b) finds that in Seyfert nuclei the widths of the nebular lines of the NLR primarily reflect the gravitational potential of the central region of the host galaxy. He argues that those objects having line velocities that exceed the virial prediction have an additional acceleration mechanism, most likely in the form of radio jets emanating from the nucleus. Do those LINERs whose line widths are larger than those in Seyferts fall in this category? One might further speculate that perhaps shocks generated as a consequence of this ``extra'' mechanical energy source really are responsible for the spectral differences between this subset of LINERs and Seyferts. If shown to be true, this would be the ultimate vindication for the proponents of shock models (see the review by Dopita in these proceedings)! The conjecture that the NLRs of some LINERs experience additional acceleration from radio jets can be tested with appropriate radio continuum observations. An alternative possibility is that the line width differences actually reflect differences in the central mass concentration. Future models of LINERs need to take all of these factors into consideration.