High Metallicities in QSOs

Summary

Abstract

We describe constraints on the metallicity of quasar broad line region gas. The overall emission line spectrum is surprisingly insensitive to order of magnitude changes in the global metallicity Z. Indirect methods, employing photoionization models and explicit stellar chemical evolution models of the selective enrichment of the elements, must be used to infer the metal enrichment. Two line ratios, both involving NV λ1240, are developed to measure Z. The first is the ratio of NV to the collisionally excited line CIV λ1549. The second is the ratio of NV to the recombination line HeII λ1640. Both indicate nitrogen enhancements exceeding an order of magnitude above solar. These results imply high metallicities in high redshift quasars, a property they have in common with the cores of massive galaxies.

Introduction

One of the longest standing goals of AGN emission line research has been to use these lines to measure the composition of the emitting gas. This could then test models of both the quasar phenomenon and stellar nucleosynthesis (Davidson and Netzer 1979; Shields 1976). Fundamental uncertainties concerning the nature and geometry of the BLR have made this work difficult.

Variability studies have rejuvenated interest in the BLR by providing methods to directly measure quantities such as the distance between continuum source and emitting clouds. These have shown that the clouds are both denser and exposed to a far more intense radiation field than had been inferred (Peterson 1992; Ferland and Persson 1989; Ferland et al. 1992).