Annu. Rev. Astron. Astrophys. 1999. 37: 487-531
Copyright © 1999 by . All rights reserved

Reprinted with kind permission from , 4139 El Camino Way, Palo Alto, California, USA

For a PDF version of the article, click here.
For a Postscript version of the article, click here.


ELEMENTAL ABUNDANCES IN QUASISTELLAR OBJECTS:
Star Formation and Galactic Nuclear Evolution at High Redshifts

Fred Hamann


Department of Astronomy, University of Florida, 211 Bryant Space Sciences Center, Gainesville, FL 32611-2055; e-mail: hamann@astro.ufl.edu
and
Center for Astrophysics and Space Sciences, University of California, San Diego, La Jolla, California 92093-0424

Gary Ferland


Department of Physics and Astronomy, University of Kentucky, Lexington, KY 40506-0055; e-mail: gary@pa.uky.edu
and
Canadian Institute for Theoretical Astrophysics, University of Toronto, Toronto, ON, M5S 3H8 Canada


Abstract. Quasar (QSO) elemental abundances provide unique probes of high-redshift star formation and galaxy evolution. There is growing evidence from both the emission and intrinsic absorption lines that QSO environments have roughly solar or higher metallicities out to redshifts >4. The range is not well known, but solar to a few times solar metallicity appears to be typical. There is also evidence for higher metallicities in more luminous objects and for generally enhanced N/C and Fe / alpha abundances compared with solar ratios.

These results identify QSOs with vigorous, high-redshift star formation - consistent with the early evolution of massive galactic nuclei or dense protogalactic clumps. However, the QSOs offer new constraints. For example, (a) most of the enrichment and star formation must occur before the QSOs "turn on" or become observable, on time scales of ltapprox 1 Gyr at least at the highest redshifts. (b) The tentative result for enhanced Fe/alpha suggests that the first local star formation began at least ~ 1 Gyr before the QSO epoch. (c) The star formation must ultimately be extensive to reach high metallicities; that is, a substantial fraction of the local gas must be converted into stars and stellar remnants. The exact fraction depends on the shape of the initial mass function (IMF). (d) The highest derived metallicities require IMFs that are weighted slightly more toward massive stars than in the solar neighborhood. (e) High metallicities also require deep gravitational potentials. By analogy with the well-known mass-metallicity relation among low-redshift galaxies, metal-rich QSOs should reside in galaxies (or protogalaxies) that are minimally as massive (or as tightly bound) as our own Milky Way.

Key words: quasars, metallicity, emission lines, absorption lines, cosmology


Table of Contents

INTRODUCTION

EMISSION LINE DIAGNOSTICS
Overview
Origin of the Broad Emission Lines
Strategies for Abundance Work
Basics of Abundance Analysis
Collisionally-Excited Lines
Recombination Lines
Deriving Abundance Ratios
Photoionization Simulations
Abundance Diagnostics and Results

ABSORPTION LINE DIAGNOSTICS
Overview: Types of Absorption Lines
General Abundance Analysis
Broad Absorption Line Results
Narrow Absorption Line Results

GENERAL ABUNDANCE SUMMARY

ENRICHMENT SCENARIOS
Occam's Razor: The Case for Normal Galactic Chemical Evolution

MORE INSIGHTS FROM GALACTIC CHEMICAL EVOLUTION
The Galactic Mass-Metallicity Relation
Specific Abundance Predictions
Fe/alpha as a Clock
Nitrogen Abundances

IMPLICATIONS OF QSO ABUNDANCES
High-Redshift Star Formation
Fe/alpha : Timescales and Cosmology
Comparisons to Other Results

FUTURE PROSPECTS

REFERENCES

Next