[Paper Review] The VLT-UVES survey for molecular hydrogen in high-redshift damped Lyman-alpha systems

C. Ledoux, P. Petitjean+1 more|CERN Bulletin|Feb 28, 2003

Astrophysics and Star Formation StudiesPhysics and Astronomy197 citations

TL;DR

This study presents the VLT-UVES survey for molecular hydrogen in high-redshift damped Lyman-alpha systems, detecting H₂ in 13–20% of 33 systems at z > 1.8. It finds that H₂ is predominantly found in systems with high metallicities and large dust depletion factors, indicating a strong link between molecular content and dust presence, with molecular fractions typically log f < -1 and often as low as log f < -6, suggesting suppressed H₂ formation due to high temperatures or intense UV radiation.

ABSTRACT

We have searched for molecular hydrogen in damped Lyman-alpha (DLA) and sub-DLA systems at z&gt;1.8 using UVES at the VLT. Out of the 33 systems in our sample, 8 have firm and 2 have tentative detections of associated H2 absorption lines. Considering that 3 detections were already known from past searches, H2 is detected in 13 to 20 percent of the newly-surveyed systems. We report new detections of molecular hydrogen at z=2.087 and 2.595 toward, respectively, Q 1444+014 and Q 0405-443, and also reanalyse the system at z=3.025 toward Q 0347-383. We find that there is a correlation between metallicity and depletion factor in both our sample and also the global population of DLA systems (60 systems in total). The DLA and sub-DLA systems where H2 is detected are usually amongst those having the highest metallicities and the largest depletion factors. Moreover, the individual components where H2 is detected have depletion factors systematically larger than other components in the profiles. In two different systems, one of the H2-detected components even has [Zn/Fe]&gt;=1.4. These are the largest depletion factors ever seen in DLA systems. All this clearly demonstrates the presence of dust in a large fraction of the DLA systems. The mean H2 molecular fraction is generally small in DLA systems and similar to what is observed in the Magellanic Clouds. From 58 to 75 percent of the DLA systems have log f1000 K) and/or the ionizing flux is enhanced relative to what is observed in our Galaxy.

Motivation & Objective

• To search for molecular hydrogen (H₂) in high-redshift damped Lyman-alpha (DLA) and sub-DLA systems using high-resolution spectroscopy.
• To determine the relationship between H₂ detection, metallicity, and dust depletion in DLA systems.
• To assess the role of dust and UV radiation in suppressing H₂ formation in high-redshift interstellar media.
• To compare the physical conditions in H₂-detected systems with the broader DLA population and with the Magellanic Clouds.
• To evaluate whether DLA samples are biased against systems with high dust content and high metallicities.

Proposed method

• Utilized the UVES echelle spectrograph on the VLT at Paranal Observatory to obtain high-resolution spectra of 33 high-redshift DLA and sub-DLA systems.
• Performed absorption line fitting to detect H₂ rotational transitions with a detection limit of N(H₂) ≈ 2×10¹⁴ cm⁻².
• Measured metallicity [X/H] using Zn, S, or Si as reference elements and derived depletion factors [X/Fe] for Fe and other elements.
• Calculated the molecular fraction f = 2N(H₂)/[2N(H₂) + N(H i)] to assess the relative abundance of molecular over atomic hydrogen.
• Compared H₂-detected components with non-detections within the same systems to assess local conditions.
• Analyzed the correlation between metallicity, depletion factors, and H₂ detection across the sample and the global DLA population (60 systems).

Experimental results

Research questions

• RQ1 Is there a correlation between H₂ detection and metallicity or dust depletion in high-redshift DLA systems?
• RQ2 What physical conditions (e.g., temperature, UV field) explain the low molecular fractions observed in most DLA systems?
• RQ3 Do DLA systems with high H₂ content exhibit systematically higher depletion factors and metallicities than the general DLA population?
• RQ4 To what extent is the DLA sample biased against systems with high dust content and high metallicities?
• RQ5 How do the H₂ abundances in DLA systems compare to those in the Magellanic Clouds and the Milky Way disk?

Key findings

• Molecular hydrogen was detected in 8 systems with firm detections and 2 with tentative detections, representing 13–20% of the newly surveyed sample.
• H₂ is predominantly found in systems with the highest metallicities and largest depletion factors, with all five systems having the largest [Zn/Fe] values showing H₂ detections.
• The component with the highest depletion factor in one system reached [Zn/Fe] ≥ 1.4, the largest ever observed in a DLA system.
• The mean molecular fraction f is typically log f < -1, and 58–75% of DLA systems have log f < -6, indicating very low H₂ abundance relative to atomic hydrogen.
• There is no correlation between H₂ abundance and H i column density, as two H₂-detected systems have log N(H i) < 20.3, classifying them as sub-DLAs.
• The low molecular fractions are consistent with suppressed H₂ formation due to high gas temperatures (T > 1000 K) and/or enhanced UV radiation fields, similar to conditions in the LMC and SMC.