Quasars as the formation sites of high-redshift ellipticals: a signature in the 'associated' absorption-line systems?

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Abstract
Published data on the average metallicities and abundance ratios for absorption-line systems in high-redshift quasars suggest that a dichotomy may exist between the chemical composition of damped Lyman α (Lyα) systems (interpreted as intervening galaxies in the QSO line of sight) and the zabs ≃zem absorption-line systems associated with the quasar. Intervening systems have smaller than solar metallicities, whereas associated absorbers have solar or greater than solar metallicities and small N/C ratios. While these results have to be confirmed by more precise abundance determinations, we argue that they may be explained by an early phase of efficient metal enrichment occurring only in the close environment of high-z QSOs, and characterized by an excess type-II supernova (SNII) activity. This is reminiscent of the SNII phase required to explain the abundance ratios (favouring a- over Fe-group elements) observed in the intracluster (IC) medium of local galaxy clusters. We explore the following scenario, to be tested by forthcoming observations of QSO absorption lines using very large optical telescopes, (a) Well-studied damped-Lya, Lya and metal lines in intervening systems trace only part of the history of metal production in the Universe — the one concerning slowly star-forming discs or dwarf irregulars, (b) The complementary class of early-type and bulge-dominated galaxies formed quickly (at z ≿ 4–5) through a huge episode of star formation favouring high-mass stars, (c) The nucleus of the latter is the site of the subsequent formation of a quasar, which partly hides from view the dimmer host galaxy, (d) The products of a galactic wind, following the violent episode of star formation in the host galaxy and metal pollution of the IC medium in the forming cluster, could be directly observable in the zabs≃ zemassociated absorption systems on the QSO line of sight.

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