Metals in the intergalactic medium

We use high spectral resolution (R = 45 000) and high signal-to-noise ratio (S/N similar to 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 tau(H I) > 1. Our observations strongly suggest that the C iv/H I ratio decreases with decreasing tau(HI) with log tau(CIV) = 1.3 x log tau(H I) - 3.2. We do not detect C IV absorption statistically associated with gas of tau(H I) < 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 0 vi down to τ(H I) ∼ 0.2 with log τ(OVI)/τ(H I) ∼ 2.0. We show that 0 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.