Abstract

We have searched for molecular hydrogen in damped Lyman ? (DLA) and sub-DLA systems at high redshift (zabs > 1.8) using the Ultraviolet and Visible Echelle Spectrograph (UVES) at the Very Large Telescope (VLT) down to a detection limit of typically N (H2) = 2 × 1014 cm-2. Out of the 33 systems in our sample, eight have firm and two have tentative detections of associated H2 absorption lines. Considering that three detections were already known from past searches, molecular hydrogen is detected in 13-20 per cent of the newly surveyed systems. We report new detections of molecular hydrogen at z abs = 2.087 and 2.595 toward, respectively, Q 1444+014 and Q 0405-443, and also reanalyse the system at zabs = 3.025 toward Q 0347-383. In all of the systems, we measure metallicities relative to solar, [X/H] (with either X = Zn, S or Si), and depletion factors of Fe, [X/Fe], supposedly on to dust grains, and compare the characteristics of our sample with those of the global population of DLA systems (60 systems in total). We find that there is a correlation between the metallicity and the depletion factor in both our sample and also the global population of DLA systems. Although HZ molecules are detected in systems with [Zn/Fe] as small as 0.3, the DLA and sub-DLA systems where H2 is detected are usually amongst those having the highest metallicities and the largest depletion factors. In particular, H2 is detected in the five systems having the largest depletion factors. Moreover, the individual components where H2 is detected have depletion factors systematically larger than other components in the profiles. In two different systems, one of the H2-detected components even has [Zn/Fe]?1.4. These are the largest depletion factors ever seen in DLA systems. All of this clearly demonstrates the presence of dust in a large fraction of the DLA systems. The mean H2 molecular fraction, f = 2N(H2)/[2N(H2) + N(H I)], is generally small in DLA systems (typically log f < -1) and similar to what is observed in the Magellanic Clouds. There is no correlation between the observed amount of H2 and the H I column density. In fact, two systems where H 2 is detected have log N(HI) < 20.3 and, therefore, are sub-DLA systems. From 58 to 75 per cent of the DLA systems have log f < -6. This can be explained if the formation rate of H2 on to dust grains is reduced in those systems, probably because the gas is warm (T > 1000 K) and/or the ionizing flux is enhanced relative to what is observed in our Galaxy.