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
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
Below are some specific issues that new studies might address.
- More data at high redshifts will constrain better the
epoch and extent of early star formation associated with QSOs.
- Reliable measurements of Fe /
will further constrain
the epoch of first star formation and, perhaps, the cosmology via the
~ 1 Gyr enrichment clock.
- 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.
- 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.
- 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?
- 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?
- 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.
- 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.
- 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.
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
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