Metals in the intergalactic medium ^*

¹ Institut d'Astrophysique de Paris – CNRS, 98bis Boulevard Arago, 75014 Paris, France
² LERMA, Observatoire de Paris-Meudon, 61 avenue de l'Observatoire, 75014 Paris, France e-mail: petitjean@iap.fr
³ Observatoire de Strasbourg, 11 rue de l'Université, 67000 Strasbourg, France

Corresponding author: B. Aracil, aracil@iap.fr

Received: 18 September 2003
Accepted: 15 February 2004

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.