Quasar (or QSO) elemental abundances provide unique probes of high-redshift
star formation and galaxy evolution. There is growing evidence from both the
emission and intrinsic absorption lines that QSO environments have roughly
solar or higher metallicities out to redshifts >4. The range is not well known,
but solar to a few times solar appears to be typical. There is also evidence
for higher metallicities in more luminous objects, and for generally enhanced
N/C and Fe/alpha abundances compared to solar ratios.
These results identify QSOs with vigorous, high-redshift star formation --
consistent with the early evolution of massive galactic nuclei or dense
proto-galactic clumps. However, the QSOs offer new constraints. For example, 1)
most of the enrichment and star formation must occur before the QSOs "turn on"
or become observable, on time scales of ~<1 Gyr at least at the highest
redshifts. 2) The tentative result for enhanced Fe/alpha suggests that the
first local star formation began at least ~1 Gyr prior to the QSO epoch. 3) The
star formation must ultimately be extensive in order to reach high
metallicities, i.e. a substantial fraction of the local gas must be converted
into stars and stellar remnants. The exact fraction depends on the shape of the
initial mass function (IMF). 4) The highest derived metallicities require IMFs
that are weighted slightly more toward massive stars than the in solar
neighborhood. 5) High metallicities also require deep gravitational potentials.
By analogy with the well-known mass--metallicity relation among low-redshift
galaxies, metal-rich QSOs should reside in galaxies (or proto-galaxies) that
are minimally as massive (or as tightly bound) as our own Milky Way.Comment: To appear in the 1999 Annual Review of Astronomy and Astrophysics, 47
pages, 13 figures (3-6 and 12 not embedded
