Abstract

Aims. We present the current status of ongoing searches for molecular hydrogen in high-redshift (1.8 < 4.2) Damped Lyman-α systems (DLAs) capitalising on observations performed with the ESO Very Large Telescope (VLT) Ultraviolet and Visual Echelle Spectrograph (UVES). Methods. We identify 77 DLAs/strong sub-DLAs, with log (H I) 20 and >1.8, which have data that include redshifted H Lyman and/or Werner-band absorption lines. This sample of H I, H and metal line measurements, performed in an homogeneous manner, is more than twice as large as our previous sample (Ledoux et al. 2003) considering every system in which searches for H could be completed so far, including all non-detections. Results. H is detected in thirteen of the systems, which have molecular fractions of values between 510 and 0.1, where = 2(H)/(2(H)+N(H I)). Upper limits are measured for the remaining 64 systems with detection limits of typically log (H)∼14.3, corresponding to log < -5. We find that about 35% of the DLAs with metallicities relative to solar [X/H] -1.3 (i.e., 1/20th solar), with X = Zn, S or Si, have molecular fractions log > -4.5, while H is detected - regardless of the molecular fraction - in 50% of them. In contrast, only about 4% of the [X/H] < -1.3 DLAs have log > -4.5. We show that the presence of H does not strongly depend on the total neutral hydrogen column density, although the probability of finding log > -4.5 is higher for log (H I) 20.8 than below this limit (19% and 7% respectively). The overall H detection rate in log (H I) 20 DLAs is found to be about 16% (10% considering only log > -4.5 detections) after correction for a slight bias towards large (H I). There is a strong preference for H-bearing DLAs to have significant depletion factors, [X/Fe] > 0.4. In addition, all H-bearing DLAs have column densities of iron into dust grains larger than log ∼ 14.7, and about 40% of the DLAs above this limit have detected H lines with log > -4.5. This demonstrates the importance of dust in governing the detectability of H in DLAs. Our extended sample supports neither the redshift evolution of the detection fraction of H-bearing DLAs nor that of the molecular fraction in systems with H detections over the redshift range 1.8 < 3. . © 2008 ESO.