The Evolution of Quasar Host Galaxies

Summary

ABSTRACT

Analysis of quasar broad emission lines suggests that the emitting gas is substantially enriched, often well above solar at high redshifts. The abundances are like those expected in the cores of massive galaxies early in their evolution, suggesting that observable quasars occur near the end of the epoch when rapid star formation, dominated by high mass stars, has created an enriched interstellar medium. An increase in the derived metallicities with both redshift and luminosity suggests that there is a mass-metallicity relation among quasars analogous (or identical) to the mass-metallicity relation in elliptical galaxies. This relation is consistent with the most massive quasars and/or host galaxies forming only at high redshifts.

INTRODUCTION

Observations of strong metallic emission lines in quasars out to redshift ∼4.9 imply some enrichment at times when the Universe was less than 10% of its present age. If quasars reside in the cores of massive galaxies, their gaseous environments could easily have larger than solar abundances, even at the highest redshifts. Observations and models of giant elliptical galaxies show that metallicities of at least several Z_⊙ are attained in less than 1 Gyr (cf. Arimoto and Yoshii 1987). In the bulge of our own galaxy the stellar metallicities also reach at least a few Z_⊙ (Rich 1988) and the enrichment is again believed to occur in ≲ Gyr (Köppen and Arimoto 1990). Since the gas in any evolving star cluster is as chemically enriched as the most recently formed stars, and thus more enriched than the bulk of the stellar population, metallicities above solar may be typical of the gas in massive galactic nuclei.