2004A&A...419..811A

We use high spectral resolution (R=45000) and high signal-to-noise ratio (S/N ∼35-70 per pixel) spectra of 19 high-redshift (2.1<z_em<3.2) 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 τ_ HI_>1. Our observations strongly suggest that the CIV/HI ratio decreases with decreasing τ_HI with logτ_CIV=1.3xlogτ_HI-3.2. We do not detect CIV absorption statistically associated with gas of τ_HI<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 OVI down to τ_HI∼0.2 with logτ_OVI/τ_HI~-2.0. We show that OVI absorption in the lowest density gas is located within ∼300km/s of strong HI lines. This suggests that this OVI 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.