EVOLUTIONARY PROPERTIES OF LY ALPHA-CLOUDS

The results of a systematic analysis of the redshift distribution of L y alpha clouds are presented. Our Ly alpha sample contains high-resolu tion spectra of eight quasars, which are carefully searched for the pr esence of possible metal lines. Several new metal lines are found. By using the maximum likelihood method, we calculate the evolutionary ind ex, gamma, for various values of the minimum limiting rest equivalent width/column density for the sample. Several statistical tests are per formed in order to check the significance of the fit. The gamma values obtained are less than those obtained from intermediate-resolution ob servations. We do not find very significant differential evolution, th ere being only a slight trend to strong lines evolving faster, contrar y to the trend reported in the literature. Various possibilities are c onsidered in order to understand this difference. Ly alpha lines are s hown to be clustered on scales approximately 200 km s-1. 'Poissonian s amples', constructed to eliminate the effect of clustering on evolutio nary properties, show more pronounced differential evolution, with str onger lines evolving faster. We do not find any clear evidence for the presence of the proximity effect in our sample. The redshift distribu tion appears to be somewhat flatter at low redshifts than that at high redshifts; however, a much larger data set is necessary to confirm th is result.