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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 & Yoshii 1987; Köppen & Arimoto 1990). 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 Zsmsun (e.g. Rich 1988, 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). Individual stars 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, Tinsley 1980, Rich 1990), predict that the gas was ~ 2 to 3 times more metal-rich than the current stellar means, or ~ 2 to 9 Zsmsun near the end of the main star-forming epoch. We might therefore expect QSO metallicities up to ~ 9 Zsmsun 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).