Abundances in the Lyman--alpha clouds

Abstract

We have re--examined the chemical composition of \lya clouds using the composite--cloud technique, in which each \lya line in a spectrum is shifted to its rest frame wavelength and all rest frame spectra are co--added to form an `averaged' \lya cloud spectrum. We illustrate how various estimates of the spectrum and redshift evolution of the background ionizing UV flux lead to very different predictions for the relative strengths of heavy element lines in the forest clouds. We also show how the potential abundance limits depend on the various observational quantities and how different procedures may be required for OVI and CIV. Preliminary results are presented from an analysis of two high redshift QSO echelle spectra.