Annu. Rev. Astron. Astrophys. 1999. 37: 487-531 Copyright © 1999 by Annual Reviews. All rights reserved

8. FUTURE PROSPECTS

We now have the observational and theoretical abilities to test and dramatically extend all of the QSO abundance work discussed above. The most pressing needs are to (a) develop more independent abundance diagnostics and (b) obtain more and better data to compare diagnostics in large QSO samples - spanning a range of redshifts, luminosities, radio properties, etc. Absorption line studies will benefit generally from higher spectral resolutions and wider wavelength coverage, providing more accurate column densities and more numerous constraints on the coverage fractions, ionizations, and abundances (Section 3). BEL studies should include more of the weaker lines, such as OVI 1034, CIII 977, NIII 991, and the intercombination lines, whenever possible (Section 2). Theoretical analysis of the FeII/MgII emission ratios, in particular, is needed to test the tentative conclusion for high Fe/Mg abundances. This and other BEL results should be tested further by examining the same lines (or same elements) in intrinsic NAL systems. The steady improvement in our observational capabilities at all wavelengths will provide many more diagnostic opportunities.

Below are some specific issues that new studies might address.

1. More data at high redshifts will constrain better the epoch and extent of early star formation associated with QSOs.
2. Reliable measurements of Fe / will further constrain the epoch of first star formation and, perhaps, the cosmology via the ~ 1 Gyr enrichment clock.
3. Better estimates of the metal-to-metal ratios generally will reveal more specifics of the star formation histories, via comparisons to well-studied galactic environments and theoretical nucleosynthetic yields.
4. Abundances for QSOs spanning a wide range of luminosities and redshifts will isolate any evolutionary (redshift) trends and test the tentative luminosity-Z relationship. This relationship might prove to be a useful indicator of the total masses or densities of the local stellar populations by analogy with the mass-Z trend in nearby galaxies.
5. The range of QSO metallicities at a given redshift and luminosity will help constrain the extent of star formation occurring before QSOs turn on or become observable. Are there any low-metallicity QSOs?
6. Combining the QSO abundances with direct-imaging studies of their host galaxies should test ideas about the chemical enrichment and help us interpret data at the highest redshifts, where direct imaging is (so far) not possible. For example, are QSOs in large galaxies (e.g. giant ellipticals) more metal-rich than others?
7. Correlations between the abundances and other properties of QSOs, such as radio loudness or UV-X-ray continuum shape, might reveal new environmental factors in the enrichment or systematic uncertainties in our abundance derivations.
8. Observations with wide-wavelength coverage would allow us to compare abundances derived from the narrow emission lines (appearing in the rest frame optical) to BEL and NAL data in the same objects. These diverse diagnostics might provide crude abundance maps of QSO host galaxies.
9. How do QSO abundances compare with their low-redshift counterparts, the Seyfert galaxies and other low-luminosity active galactic nuclei? We might find that the metallicities at low redshifts are less than the QSOs owing to recent mergers or gaseous infall.

Acknowledgments

We are grateful to G Burbidge for his help and encouragement. We also thank KT Korista, A Laor, A Sandage and JC Shields for comments on this manuscript, and TA Barlow, N Arav and VT Junkkarinen for helpful discussions. GF thanks the Canadian Institute for Theoretical Astrophysics for their hospitality during a sabbatical year, and acknowledges support from the Natural Science and Engineering Research Council of Canada through CITA. The work of FH was supported by NASA grant NAG 5-3234. Research in nebular astrophysics at the University of Kentucky is supported by the NSF through grant 96-17083 and by NASA through its ATP (award NAG 5-4235) and LTSA programs.