Properties of high-redshift Lyman-alpha clouds. I - Statistical analysis of the Schneider-Schmidt-Gunn quasars

Techniques for the statistical analysis of the Lya forest in high redshift quasars are developed, and applied to the low resolution (25 AA) spectra of 29 of the 33 quasars in the Schneider-Schmidt-Gunn (SSG) sample.We find that the mean absorption increases with $z$ approximately as a power law $(1+z)^{gamma+1}$ with $gamma = 2.46pm 0.37$. The mean ratio of Lya to Lyman $eta$ absorption in the clouds is $0.476pm 0.054$. We also detect, and obtain ratios, for Lyman $eta$, $gamma$, and possibly $epsilon$. We are also able to quantify the fluctuations of the absorption around its mean, and find that these are comparable to, or perhaps slightly larger than, that expected from an uncorrelated distribution of clouds. The techniques in this paper, which include the use of bootstrap resampling of the quasar sample to obtain estimated errors and error covariances, and a mathematical treatment of absorption from a (possibly non-uniform) stochastic distribution of lines, should be applicable to future, more extensive, data sets.