Molecular hydrogen in high-redshift Damped Lyman-α systems: The VLT/UVES database

Aims. We present the current status of ongoing searches for molecular hydrogen in high-redshift (1.8 < zabs ≤ 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 N(H i) ≥ 20 and zabs > 1.8, which have data that include redshifted H2 Lyman and/or Werner-band absorption lines. This sample of H , H2 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 H2 could be completed so far, including all non-detections. Results. H2 is detected in thirteen of the systems, which have molecular fractions of values between f ≃ 5 × 10−7 and f ≃ 0.1, where f = 2N(H2)/(2N(H2) + N(H i)). Upper limits are measured for the remaining 64 systems with detection limits of typically log N(H2) ∼ 14.3, corresponding to log f < −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 f > −4.5, while H2 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 f > −4.5. We show that the presence of H2 does not strongly depend on the total neutral hydrogen column density, although the probability of finding log f > −4.5 is higher for log N(H i) ≥ 20.8 than below this limit (19% and 7% respectively). The overall H2 detection rate in log N(H i) ≥ 20 DLAs is found to be about 16% (10% considering only log f > −4.5 detections) after correction for a slight bias towards large N(H i). There is a strong preference for H2-bearing DLAs to have significant depletion factors, [X/Fe] > 0.4. In addition, all H2-bearing DLAs have column densities of iron into dust grains larger than log N(Fe)dust ∼ 14.7, and about 40% of the DLAs above this limit have detected H2 lines with log f > −4.5. This demonstrates the importance of dust in governing the detectability of H2 in DLAs. Our extended sample supports neither the redshift evolution of the detection fraction of H2-bearing DLAs nor that of the molecular fraction in systems with H2 detections over the redshift range 1.8 < zabs ≤ 3.