This review has not, of course, covered every conceivable application of infrared spectroscopy. One major area I have omitted is the use of line velocities and profiles to study the kinematics and dynamics of astronomical sources. This type of study is not as unique to infrared lines as are some of the other topics discussed above, except in studying regions of heavy obscuration, such as the nuclei of starburst galaxies such as M82 (e.g., Achtermann & Lacy 1995). So far there has been only a limited amount of high-resolution infrared spectroscopy of emission lines, primarily due to sensitivity limitations. We can anticipate explosive growth of this area in the future, as infrared astronomers become less "signal-starved" with the advent of improved instruments and facilities. On the instrumentation side, individual detectors have already achieved high quantum efficiencies, but for spectroscopic applications, there is a tremendous advantage to using large detector arrays to obtain simultaneous coverage of a large number of spectral resolution elements.
The next decade will also see a major step forward in the light-gathering power of telescopes. On the ground, there will be several 8-10 m class telescopes in operation. Some of them more infrared-optimized than others, but all will undoubtedly be used for at least some infrared spectroscopy: Keck I & II and the Subaru Telescope, on Mauna Kea; Gemini North & South, in Hawaii & Chile; the VLT, in Chile, and the Hobby-Eberly Telescope in Texas. For spectral regions that are blocked from the ground, ESA's Infrared Space Observatory (ISO), scheduled to be launched in late 1995, will be the first space telescope to do extensive infrared spectroscopy. The premier facility for continuing infrared spectroscopic work at these wavelengths will be the Stratospheric Facility for Infrared Astronomy (SOFIA), a 3 m airborne telescope (Erickson & Davidson 1995). SOFIA's ten-fold increase in collecting area and smaller beam size, compared with the KAO, will give it the sensitivity to do infrared spectroscopy on relatively faint and distant sources, and its operational mode will encourage frequent upgrading of auxiliary instrumentation such as infrared spectrometers. At this writing, it appears likely that SOFIA will be in operation by around the year 2000. It is to be hoped that other, more advanced infrared space observatories, such as SIRTF, will follow soon after.
The utility of infrared emission line observations will also be improved by advances on the theoretical side. One major uncertainty in the interpretation of infrared lines has been the values of some of the atomic constants. Fortunately, this situation is now beginning to be remedied (e.g., Blum & Pradhan 1992). Improved models of the chemical, ionization, and thermal structure of photoionized regions, photodissociation regions, and interstellar shocks may help resolve some of the present anomalies and discrepancies that have arisen from studies of infrared emission lines, and permit stronger conclusions to be drawn on the roles of shocks and photoionization in distant galaxies.
Infrared spectroscopy is rapidly becoming a tool that is used by a large number of astronomers, rather than being the domain of a specialized few. This is likely to become even more true in the future, when facility infrared spectrometers on large telescopes are readily available to guest observers without specialized training. Thus, it is useful for all of us to be aware of the potential of infrared emission lines for nebular astrophysics. The techniques reviewed in this paper are merely the beginning of the harvest to be reaped from infrared line spectroscopy in the future.
I am grateful to the organizers of this conference for giving me the opportunity to present this review and to participate in this very special occasion, honoring two scientists who have played such a major role in advancing our understanding of the properties and physics of nebulae. I also wish to acknowledge research support from NSF grant AST 91-15101 and HST grant GO 3880.01-91A during the preparation of this review.