A&A 481, 327-336 (2008)

DOI: 10.1051/0004-6361:20078780

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

**P. Noterdaeme**

^{1, 2}, C. Ledoux^{1}, P. Petitjean^{2}, and R. Srianand^{3}^{1}European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Vitacura, Santiago 19, Chile

e-mail: [pnoterda;cledoux]@eso.org

^{2}UPMC Paris 6, Institut d'Astrophysique de Paris, CNRS, 98bis boulevard Arago, 75014 Paris, France

e-mail: petitjean@iap.fr

^{3}Inter-University Centre for Astronomy and Astrophysics, Post Bag 4, Ganesh Khind, Pune 411007, India

e-mail: anand@iucaa.ernet.in

(Received 2 October 2007 / Accepted 17 January 2008)

** Abstract ***Aims. *
We present the current status of ongoing searches for molecular
hydrogen in high-redshift (1.8 <
4.2) Damped Lyman-
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
>1.8, which have data that include redshifted H_{2} Lyman
and/or Werner-band absorption lines. This sample of H I, H_{2}
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_{2} could be
completed so far, including all non-detections.
*Results. *
H_{2} is detected in thirteen of the systems, which have molecular
fractions of values between *f* 510^{-7} and *f*
0.1, where *f* = 2*N*(H_{2})/(2*N*(H_{2})+N(H I)). Upper limits are
measured for the remaining 64 systems with detection limits of
typically log *N*(H_{2})~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 H_{2} 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 H_{2} 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 H_{2} 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
H_{2}-bearing DLAs to have significant depletion factors,
[X/Fe] > 0.4. In addition, all H_{2}-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_{2} lines with log *f* > -4.5. This demonstrates the
importance of dust in governing the detectability of H_{2} in DLAs.
Our extended sample supports neither the redshift evolution of the
detection fraction of H_{2}-bearing DLAs nor that of the molecular
fraction in systems with H_{2} detections over the redshift range
1.8 <
3.

**Key words:**cosmology: observations

**--**galaxies: quasars: absorption lines

**--**galaxies: ISM

**--**ISM: molecules

**©**

*ESO 2008*