Quasar absorption lines: The chemical composition of the absorbing clouds

O.J.Dittmann, J.Köppen

ABSTRACT Using photoionisation models we present an algorithm for the analysis of spectra of QSO absorption systems. Tests with artificially created noisy systems show that this method yields reliable solutions for the ionisation parameter u and the metallicity Z, and good estimates for the column density of neutral hydrogen N_H and the Doppler parameter b. We derive these four parameters for 112 absorption systems in a redshift range of 0.31 < z < 4.13. Assuming an observational limit of equivalent widths of Wlambda = 0.1 A, we find that Mg II-systems can only be detected if they have Z ≥ 0.03 and N_H > 3 10^{18} cm^{-2}. No direct evidence of a homogenous chemical evolution of the absorbing clouds is found. Only the combination of optical data from Mg II and C IV systems and the restriction to clouds with N_H > 10^{19} cm^{-2} suggests an enrichment of metals with time. We find strong evidence that the absorption arises in the halos of galaxies. All clouds exhibit roughly the same ionisation parameter, lg(u) = -2.7, independent of redshift.