Metals in the intergalactic medium

Abstract

We use high spectral resolution () and high signal-to-noise ratio ( per pixel) spectra of 19 high-redshift () quasars to investigate the metal content of the low-density intergalactic medium using pixel-by-pixel procedures. This high quality homogeneous survey gives the possibility to statistically search for metals at H i optical depths smaller than unity. We find that the gas is enriched in carbon and oxygen for neutral hydrogen optical depths > 1. Our observations strongly suggest that the C iv/H i ratio decreases with decreasing with . We do not detect C iv absorption statistically associated with gas of < 1. However, we observe that a small fraction of the low density gas is associated with strong metal lines as a probable consequence of the IGM enrichment being highly inhomogeneous. We detect the presence of O vi down to ~ 0.2 with log / ~ -2.0. We show that O vi absorption in the lowest density gas is located within ~300 km s-1 of strong H i lines. This suggests that this O vi phase may be part of winds flowing away from overdense regions. This effect is more important at the largest redshifts (z > 2.4). Therefore, at the limit of present surveys, the presence of metals in the underdense regions of the IGM is still to be demonstrated.