1995A&A...297..671D

An algorithm for the analysis of QSO absorption line systems has been developed in which the parameters of photoionisation models are automatically adjusted until an optimal fit to the equivalent widths of all the lines observed in a system is found. Tests with artificially created noisy data show that it allows a reliable determination of the ionisation parameter u and the metallicity Z/Z_☉, and gives good estimates for the column density of neutral hydrogen N_HI and the Doppler parameter b. Photoionisation models also predict a strong selection effect for the detection of clouds. While with C IV-systems clouds are detectable (equivalent widths larger than W_λ=0.1A) for most of the Z/Z_☉-N_HI range, with Mg II-systems only the large and high-metallicity clouds can be found. We derive the four model parameters for 114 absorption systems in a redshift range of 0.31≤z_abs≤4.13. No direct evidence of a common homogenous chemical evolution of the absorbing clouds is found, though a trend would be detectable despite the noise in the data available. It is only the combination of optical data from Mg II- and C IV-systems which suggests an enrichment of metals with time. The ratio of column densities of C IV and Si IV strongly indicates that the absorption arises in the halos of galaxies. The ionisation parameter is fairly evenly distributed over in the range log(u)=-3.5 ... -0.5, suggesting that the cloud population consists of absorbers with a rather wide range of physical properties.