Abstract

Aims. We characterize the importance of metallicity on the presence of molecular hydrogen in damped Lyman-? (DLA) systems. Methods. We constructed a representative sample of 18 DLA/sub-DLA systems with log N(H I) > 19.5 at high redshift (zabs > 1.8) and with metallicities relative to solar [X/H] > -1.3 (with [X/H] = log N(X)/N(H)-log (X/H) ? 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 zabs = 2.431 toward Q2343+125 and at zabs = 2.426 toward Q2348-011. Q2348-011, From a comparison of the H2 detection fraction in the highmetallicity 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. © ESO 2006.