QUASAR ABSORPTION-LINES .1. THE CHEMICAL-COMPOSITION OF THE ABSORBINGCLOUDS

An algorithm for the analysis of QSO absorption line systems has been developed in which the parameters of photoionisation models are automa tically adjusted until an optimal fit to the equivalent widths of all the lines observed in a system is found. Tests with artificially creat ed noisy data show that it allows a reliable determination of the ioni sation parameter u and the metallicity Z/Z(.), and gives good estimate s for the column density of neutral hydrogen N-HI and the Doppler para meter b. Photoionisation models also predict a strong selection effect for the detection of clouds. While with C IV-systems clouds are detec table (equivalent widths larger than W-lambda = 0.1 Angstrom) 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 f or 114 absorption systems in a redshift range of 0.31 less than or equ al to z(abs) less than or equal to 4.13. No direct evidence of a commo n homogenous chemical evolution of the absorbing clouds is found, thou gh 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-syst ems which suggests an enrichment of metals with time. The ratio of col umn densities of C IV and Si IV strongly indicates that the absorption arises in the halos of galaxies. The ionisation parameter is fairly e venly distributed over in the range Ig u = -3.5... - 0.5, suggesting t hat the cloud population consists of absorbers with a rather wide rang e of physical properties.