2008A&A...481..327N

We present the current status of ongoing searches for molecular hydrogen in high-redshift (1.8<z_abs≤4.2) Damped Lyman-α systems (DLAs) capitalising on observations performed with the ESO Very Large Telescope (VLT) Ultraviolet and Visual Echelle Spectrograph (UVES). We identify 77 DLAs/strong sub-DLAs, with logN(HI)≥20 and z_abs>1.8, which have data that include redshifted H₂ Lyman and/or Werner-band absorption lines. This sample of HI, 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. H₂ is detected in thirteen of the systems, which have molecular fractions of values between f≃5x10^–7 and f≃0.1, where f=2N(H₂)/(2N(H₂)+N(HI)). Upper limits are measured for the remaining 64 systems with detection limits of typically logN(H₂)∼14.3, corresponding to logf←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 logf>-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 logf>-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 logf>-4.5 is higher for logN(HI)≥20.8 than below this limit (19% and 7% respectively). The overall H₂ detection rate in logN(HI)≥20 DLAs is found to be about 16% (10% considering only logf>-4.5 detections) after correction for a slight bias towards large N(HI). 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 logN(Fe)_dust∼14.7, and about 40% of the DLAs above this limit have detected H₂ lines with logf>-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<z_abs≤3.