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A&A 456, L9-L12 (2006)
DOI: 10.1051/0004-6361:20065769

Letter

Metallicity as a criterion to select H $\mathsf$-bearing damped Lyman- $\mathsf{\alpha}$ systems

P. Petitjean1, 2, C. Ledoux3, P. Noterdaeme3 and R. Srianand4

1  Institut d'Astrophysique de Paris, CNRS - Université Pierre et Marie Curie, 98bis boulevard Arago, 75014 Paris, France
    e-mail: petitjean@iap.fr
2  LERMA, Observatoire de Paris, 61 avenue de l'Observatoire, 75014 Paris, France
3  European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Vitacura, Santiago, Chile
4  IUCAA, Post Bag 4, Ganesh Khind, Pune 411 007, India

(Received 7 June 2006 / Accepted 16 July 2006)

Abstract
Aims.We characterize the importance of metallicity on the presence of molecular hydrogen in damped Lyman-$\alpha$ (DLA) systems.
Methods.We constructed a representative sample of 18 DLA/sub-DLA systems with log N(H I) > 19.5 at high redshift ( $z_{\rm abs}$ > 1.8) and with metallicities relative to solar [X/H] > -1.3 (with [X/H] = log N(X)/N(H)-log (X/H)$_{\odot}$ and X either Zn, S or Si). We gathered data covering the expected wavelength range of redshifted H2 absorption lines on all systems in the sample from either the literature (10 DLAs), the UVES-archive, or new VLT-UVES observations for four of them. The sample is large enough to allow us to discuss for the first time the importance of metallicity as a criterion for the presence of molecular hydrogen in the neutral phase at high-z.
Results.From the new observations, we report two new detections of molecular hydrogen in the systems at $z_{\rm abs} = 2.431$ toward Q 2343+125 and at $z_{\rm abs}= 2.426$ toward Q 2348-011. Q 2348-011, From a comparison of the H2 detection fraction in the high-metallicity sample with earlier results, we find that the fraction of DLA systems with log f = log 2N(H2)/(2N(H2) + N(H I)) > -4 is as large as 50% for [X/H] > -0.7, when it is only ~5% for [X/H] < -1.3 and ~15% in the overall sample (with -2.5 < [X/H] < -0.3). This demonstrates that the presence of molecular hydrogen at high redshift is strongly correlated with metallicity.


Key words: galaxies: high-redshift -- galaxies quasars: absorption lines -- galaxies quasars: individual: Q 2343+125 -- galaxies quasars: individual: Q 2348-011



© ESO 2006


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