2006A&A...456L...9P

We characterize the importance of metallicity on the presence of molecular hydrogen in damped Lyman-α (DLA) systems. We constructed a representative sample of 18 DLA/sub-DLA systems with log N(HI)>19.5 at high redshift (z_abs>1.8) and with metallicities relative to solar [X/H]>-1.3 (with [X/H]=logN(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. From the new observations, we report two new detections of molecular hydrogen in the systems at z_abs=2.431 toward Q2343+125 and at z_abs=2.426 toward Q2348-011. Q2348-011, 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=log2N(H₂)/(2N(H₂)+N(HI))>-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.