Molecular hydrogen in high-redshift damped Lyman-αsystems: the VLT/UVES database

Abstract

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 log N(H I) ≥20 and z_abs>1.8, which have data that include redshifted H₂ Lyman and/or Werner-band absorption lines. This sample of H I, 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 ⋍ 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 <z_abs≤ 3.