The Chemical Evolution of QSOs

Summary

Abstract

One zone chemical evolution models are developed for application to QSO broad emission line regions. The elemental abundances derived from the broad line ratios imply that the gas is highly evolved, with metallicities ranging from ∼1 to ≳ 10 times solar. The short timescales (i.e. ≲ 1 Gyr if q_o ≈ ½) and relatively flat initial mass functions (compared to the solar neighbourhood) needed to fit most of the high redshift line ratios are almost identical to the parameters used in one zone models of elliptical galaxies. We conclude that the QSO phenomenon generally follows an episode of rapid star formation, exactly like that expected in massive, young galactic nuclei.

An observed trend in the emission line data suggesting higher metallicities at high redshifts could result from a mass–metallicity–redshift relation among the QSOs. Thus the highest mass QSOs (and/or host galaxies) might form only at high redshifts (e.g. z > 2).

Introduction

The broad emission line spectra of QSOs show that heavy elements are present at redshifts up to nearly z ∼ 5. Therefore some amount of star formation must have occurred before the QSOs ‘turned on’. Unfortunately, the line strengths are not indicative of the overall metal content of the gas, but some of the line ratios are sensitive to the relative abundances (see Ferland & Hamann this volume). The relative abundances can in turn be used to constrain both the metallicity and the past evolution because the elements form by different processes and on different timescales; cf. [2].