Chemical abundances in QSO host galaxies and environments from narrow absorption line systems

Abstract

We determined C, N and a-element relative abundances in the gas surrounding six quasi-stellar objects (QSOs) at an average redshift of z 2.4, by studying six narrow associated absorption systems in Ultraviolet Visual Echelle Spectrograph (UVES) high-resolution spectra. We found five systems with a metallicity (measured by C/H) consistent or above the solar value. The ionization structure observed in the associated systems is clearly different from that of the intervening ones, indicating that the associated systems are influenced by the strong UV flux from the QSO. There is a possible correlation (anticorrelation) between [N/C] ([Si/C]) and [C/H] of the studied associated systems, and [N/C]= 0 when [C/H]= 0. We have compared these observational results with the predictions of a model simulating the joint evolution of QSOs and their spheroidal hosts. The agreement turns out to be very good; in particular, the case envisaging massive haloes and high star formation rates recovers both the correlation between [N/C] and [C/H] and the anticorrelation for [Si/C] versus [C/H]. Narrow associated absorption systems prove to be powerful tracers of the chemical abundances in gas belonging to high-redshift spheroidal galaxies. The outflow of this same gas, triggered by the QSO feedback, is probably going to contribute to the early enrichment of the surrounding intergalactic medium. A larger sample, possibly increasing the number of ionization stages, chemical elements and the redshift range, would allow us to put firm constraints on detailed chemical evolution models of galaxies at high redshifts.