The Very Large Telescope ultraviolet and Visible Echelle Spectrograph survey for molecular hydrogen in high-redshift damped Lyman α systems

We have searched for molecular hydrogen in damped Lyman alpha (DLA) and sub-DLA systems at high redshift (Z(abs) > 1.8) using the Ultraviolet and Visible Echelle Spectrograph (UVES) at the Very Large Telescope (VLT) down to a detection limit of typically N(H-2) = 2 x 10(14) cm(-2). Out of the 33 systems in our sample, eight have firm and two have tentative detections of associated H-2 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 z(abs) 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 H-2 molecules are detected in systems with [Zn/Fe] as small as 0.3, the DLA and sub-DLA systems where H-2 is detected are usually amongst those having the highest metallicities and the largest depletion factors. In particular, H-2 is detected in the five systems having the largest depletion factors. Moreover, the individual components where H-2 is detected have depletion factors systematically larger than other components in the profiles. In two different systems, one of the H-2-detected components even has [Zn/Fe] greater than or equal to 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 H-2 molecular fraction, f = 2N(H-2)/[2N(H-2) + 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 H-2 and the H I column density. In fact, two systems where H-2 is detected have log N(H I) < 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 H-2 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.