As much as observations at other wavelengths are opening new doors to our understanding of LINERs, I hope that this review has persuaded the reader that the more conventional technique of optical spectroscopy still has much to offer. In the same spirit, I will confine my remarks on future work from an optical perspective.
Aside from simple statements concerning the morphological types of the host galaxies of LINERs and Seyferts, one can refine the treatment considerably by considering more quantitative measures of the bulge luminosity. This can be achieved in a straightforward manner from careful bulge/disk decomposition (Kormendy 1977; Boroson 1981), particularly as applied to modern broad-band CCD images. Sizable data bases are rapidly becoming available for many nearby galaxies (e.g., Prieto et al. 1992; de Jong 1996; Frei et al. 1996), but a concerted effort to obtain photometry for the entire Palomar sample would be highly desirable, and such a program is under execution. An even more relevant parameter to consider is the mass of the host galaxy on the relevant scales; for the scale of the bulge and even smaller, this can be accomplished by measuring optical rotation curves in the traditional manner (e.g., Rubin, Whitmore, & Ford 1988).
With the successful identification of the large number of LINERs and
low-luminosity AGNs from ground-based surveys, we should now focus on
identifying the key parameters in a given galaxy that regulate the level of
activity observed. Why do some galaxy nuclei emit such feeble power compared
to others? Is the mass of the central object smaller, is the accretion rate
(relative to the Eddington rate) curtailed, or perhaps some combination of the
two? The amount of gaseous material required to sustain the modest observed
luminosities of most nearby AGNs is quite small, being typically much less
than 1 M
yr-1. Such a fueling rate conceivably can be maintained
by mass loss from normal stars
(Ho et al. 1996b),
and the general interstellar
medium from the gaseous disk must also contribute at some level; hence, it
appears that the availability of fuel should not be a major factor. It is
possible, of course, that the accretion process itself is very inefficient,
thereby resulting in a low accretion rate. On the other hand, if the mass of
the central object plays the decisive role, then kinematic observations can be
used to provide the test. Although high-precision ground-based optical
spectroscopy has been used to hunt for massive black holes in the centers of
galaxies
(Kormendy & Richstone
1995),
atmospheric seeing inherently limits
the spatial resolution, and the results, although highly suggestive in several
instances, are not conclusive. This problem, of course, is tailor-made for
HST, especially
after the installation of the Space Telescope Imaging
Spectrograph (STIS).
By analogy with the Seyfert class, in which a continuous sequence of broad-line strength is seen (Osterbrock 1981), LINERs also appear to follow a similar sequence. The relative visibility of the BLR in at least some Seyferts has been successfully interpreted in terms of orientation effects (Antonucci 1993). It seems logical to infer that the same ``unification'' picture should extend to the realm of LINERs, unless some crucial element of the model (e.g., the presence or geometry of the obscuring molecular torus) should turn out to depend on the ionization state. It would be highly worthwhile to apply the techniques of optical spectropolarimetry to LINERs, even though the generally low signal levels of these nuclei render such an experiment quite challenging.
The luminosity function of AGNs at the faint end has implications for many fundamental astrophysical problems. Since LINERs constitute the bulk of the AGN population at low luminosities, determining the luminosity function of LINERs is a high priority (this work is in progress).
One obviously hopes to understand the behavior of any class of astronomical
objects as a function of time. However, obtaining optical spectra of a
representative sample of moderate-redshift galaxies for the identification of
emission-line nuclei is a formidable task. The dual need to acquire spectra
of at least moderate quality (i.e., suitable for measuring several
bright emission lines for classification) for a large sample of
galaxies dictates (at the moment) the need for a multi-object spectrograph
attached to a large (8-10 m) telescope. As an illustration of the
difficulty of such an enterprise, I mention two recent examples from the
literature. Lilly et al.
(1995,
and references therein) used the MOS-SIS
spectrograph on the CFHT (3.6 m) to obtain spectra of field galaxies at
< z > 
0.6. Despite
integration times of ~ 8
hr, most of their spectra are inadequate for our purposes. Even with the
Keck 10 m telescope, moderate-quality spectra of galaxies at intermediate
redshifts still require exposure times of ~ 1 hr
(Forbes et al. 1996).
But
despite these difficulties, there is no obvious alternative in the near future.
Fortunately, there are several suitable samples of faint galaxies from which
to choose. These include the field galaxies selected from the
HST
Medium Deep Survey
(Phillips et al. 1995),
the I
22.5 survey of
Lilly et al. (1995),
and the Las Campanas redshift survey
(Landy et al. 1996,
and references therein). Lastly, it should be borne in mind that even under
conditions of optimal seeing, 1" at z = 0.5 still projects to a linear
size of ~ 10 kpc (for H0 = 75 km s-1
Mpc-1). At these size scales,
the observed spectrum will include substantial contribution from the
integrated
light of the entire galaxy, and, depending on Hubble type, the signal from the
nucleus will be severely diluted. Thus, at intermediate redshifts, only
the brightest nuclei will be detected, and one will not be able to quantify
the faint end of the luminosity function.
The new results presented in this contribution were obtained in collaboration with Alex Filippenko and Wal Sargent and constituted a major portion of my Ph.D. thesis at U. C. Berkeley. I thank them for permission to discuss our work in advance of publication. Alex Filippenko carefully read the manuscript and provided many helpful comments. I am grateful to Anuradha Koratkar for the idea of holding this workshop, to STScI for agreeing to host it, and to the members of the local organizing committee for seeing it to fruition. My research is currently supported by a postdoctoral fellowship from the Harvard-Smithsonian Center for Astrophysics.