Metals in the Ly alpha forest

In order to probe the metallicity of the low density intergalactic medium ( IGM), we review the analysis of two exceptional quality Keck HIRES spectra. The spectra have S/N ratios 100-200 and a resolution of similar to 6 km s( -1) which permit detailed study of their intervening absorption systems. We compare the results of two techniques for determining the N(C IV)/N(H I) r atio in low column density Ly alpha clouds. Synthetic absorption spectra wi th known input metallicities are used to test the accuracy of these techniq ues and understand the significance of their results. The conclusion is tha t co-addition of absorption line regions to produce an ultra-high S/N compo site does not yet determine whether or not there is a significant drop in N (C IV)/N(H I) in low column density Ly alpha clouds. An alternative analysi s technique based on individual pixel optical depth measurements may be a m ore robust way of detecting weak absorption features.