The Chemical Evolution of QSOs and the Implications for Cosmology and Galaxy Formation

We examine the chemical evolution of QSO broad-line gas by applying spectral synthesis and chemical enrichment models to the N V/C IV and N V/He II emission-line ratios. The models indicate that BLR metallicities are typically ∼1 to perhaps ≳10 times solar. The enrichment must occur in ≲1 Gyr for sources where the redshift is ≳3 (if q0 = ½). The higher metallicity QSOs require star formation favoring massive stars (compared to the Galactic disk). These results imply that extensive evolution usually occurs before the QSOs become observable. Our models of the evolution are equivalent to models proposed for elliptical galaxies and for the bulges of disk galaxies. We conclude that the QSO phenomenon is preceded by vigorous star formation, exactly like that expected in massive, young galactic nuclei. The observed N V/C IV and N V/He II ratios can be several times larger in sources with high redshift and high luminosity. Systematically different physical conditions could contribute to these trends, but they could also result entirely from higher metallicities in the higher redshift/luminosity objects. We suggest that the high metallicities are related to higher QSO (and/or host galaxy) masses at large redshifts. This implies a mass- metallicity relation in QSOs analogous to the well-known relationship in nearby ellipticals. The trend with luminosity also suggests that metallicity differences can influence the observed "global Baldwin effect." The evolution models predict a ∼1 Gyr delay in the Fe enrichment due to Type Ia supernovae. The timescale for this delay is fixed by the (albeit uncertain) lifetimes of SN Ia precursors and is not sensitive to the IMF or star formation rates. The expected ∼1 Gyr delay could therefore be used as a clock to constrain QSO ages if accurate Fe abundances are measured. Age constraints could in turn constrain the cosmology (i.e., q0) when applied to high-redshift sources. One-zone photoionization models suggest that the delayed rise in Fe should be observable in, for example, the ratio of UV Fe II/Mg II emission lines. Most of the evolution models also predict Fe overabundances after ∼1-2 Gyr, with Fe/O and Fe/Mg up to several times solar. This overabundance might explain the strong Fe II emission observed in many QSOs and active galactic nuclei.