Metals in the Lyα Forest

Abstract

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 ∼ 6 kms^-1 which permit detailed study of their intervening absorption systems. We compare the results of two techniques for determining theN(C IV)/N(H I) ratio in low column density Lyα clouds. Synthetic absorption spectra with known input metallicities are used to test the accuracy of these techniques and understand the significance of their results. The conclusion is that co-addition of absorption line regions to produce an ultra-high S/N composite does not yet determine whether or not there is a significant drop in N(C IV)/N(H I) in low column density Lyα clouds. An alternative analysis technique based on individual pixel optical depth measurements may be a more robust way of detecting weak absorption features.