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Issue A&A
Volume 419, Number 3, June I 2004
Page(s) 811 - 819
Section Cosmology (including clusters of galaxies)
DOI http://dx.doi.org/10.1051/0004-6361:20034346



A&A 419, 811-819 (2004)
DOI: 10.1051/0004-6361:20034346

Metals in the intergalactic medium

B. Aracil1, P. Petitjean1, 2, C. Pichon3 and J. Bergeron1

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

(Received 18 September 2003 / Accepted 15 February 2004)

Abstract
We use high spectral resolution ( $R=45\,000$) and high signal-to-noise ratio ( ${\it S/N} \sim 35{-}70$ per pixel) spectra of 19 high-redshift ( $2.1<z_{\rm em}<3.2$) 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 $\tau_{\rm H\,I}$   > 1. Our observations strongly suggest that the C IV/H I ratio decreases with decreasing $\tau_{\rm H\,I}$ with $\log\,\tau_{\rm C\,IV} = 1.3\times\log\,\tau_{\rm H\,I}-3.2$. We do not detect C IV absorption statistically associated with gas of $\tau_{\rm H\,I}$  < 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 O VI down to $\tau_{\rm H\,I}$  ~ 0.2 with log  $\tau_{\rm O\,VI}$/ $\tau_{\rm H\,I}$  ~ -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.


Key words: cosmology: observations -- galaxies: halos -- galaxies: ISM -- quasars: absorption lines

Offprint request: B. Aracil, aracil@iap.fr

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