2.1. Expectations for QSO Abundances
The intrinsic emission and absorption lines of QSOs
offer direct probes of the composition and enrichment
history of the gas in these dense galactic environments.
Studies of nearby galaxies indicate that vigorous star
formation in galactic cores should have produced super-solar gas-phase
metallicities (Z) within a few billion years of the initial collapse
(e.g. Arimoto &
The enriched gas might ultimately be ejected from the
galaxy/QSO environment, consumed by the central black hole, or diluted
by subsequent gaseous infall, but the evidence for early-epoch
high-Z gas remains in the old stars today.
In particular, the mean stellar metallicities
(1) in the cores of
nearby massive galaxies (including the bulge of our own Milky
Way) are typically ~ 1 to 3 Z (e.g.
Bica et al. 1988,
Bica et al. 1990,
Gorgas et al. 1990,
Worthey et al. 1992,
McWilliam & Rich 1994,
Minniti et al. 1995,
Idiart et al. 1996,
Bruzual et al. 1997).
are distributed about these means with metallicities reflecting the
gas-phase abundance at the time of their formation. Only the most
recently formed stars at any epoch have metallicities as high as
that in the gas. Simple chemical evolution models of spheroidal
systems like elliptical galaxies or the bulges spiral disks,
wherein the gas-phase metallicity grows monotonically with time
(Searle & Zinn 1978,
predict that the
gas was ~ 2 to 3 times more metal-rich than the current stellar
means, or ~ 2 to 9 Z near the end of the main star-forming epoch.
We might therefore expect QSO metallicities up to ~ 9 Z
as long as most of the local star formation (and enrichment) occurs
before the QSOs ``turn on'' or become observable.
1 Note that ``metallicity'' is measured best from the enrichment products of massive stars like O, Mg, etc., rather than Fe (see Wheeler et al. 1989 and Section 2.3 below).