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Astron. Astrophys. 364, L26-L30 (2000)

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Letter to the Editor

Molecular hydrogen and the nature of damped Lyman-[FORMULA] systems *

P. Petitjean 1,2, R. Srianand 3 and C. Ledoux 4

1 Institut d'Astrophysique de Paris - CNRS, 98bis Boulevard Arago, 75014 Paris, France
2 UA CNRS 173 - DAEC, Observatoire de Paris-Meudon, 92195 Meudon Cedex, France
3 IUCAA, Post Bag 4, Ganeshkhind, Pune 411 007, India
4 European Southern Observatory, Karl Schwarzschild Strasse 2, 85748 Garching bei München, Germany

Received 11 August 2000 / Accepted 25 September 2000


We report results from our mini-survey for molecular hydrogen in eight high-redshift damped Lyman-[FORMULA] (DLA) systems using the ESO Ultra-violet and Visible Spectrograph on the VLT. In addition, we investigate two systems using ESO public data. We include in the sample the only system where H2 was previously detected and studied at high-spectral resolution. Altogether our sample consists of eleven absorbers with 1.85 [FORMULA] [FORMULA] [FORMULA] 3.4.

We confirm the presence of H2 in the [FORMULA] = 2.3377, metal-poor ([Si/H] = -1.20), system toward PKS 1232+082. The derived molecular fraction, f = 2N(H2)/(2N(H2)+N(H I )) = 4[FORMULA]10-4, is two orders of magnitude less than what has been claimed previously from low-resolution data. The physical conditions within the cloud can be constrained directly from observation. The kinetic temperature and particle density are in the ranges, respectively, 100 [FORMULA] T [FORMULA] 300 K and 30 [FORMULA] [FORMULA] [FORMULA] 50 cm-3. In addition, UV pumping is of the same order of magnitude than in our Galaxy.

The upper limits on the molecular fraction derived in nine of the systems are in the range 1.2[FORMULA]10-7-1.6[FORMULA]10-5. There is no evidence in this sample for any correlation between H2 abundance and relative heavy element depletion into dust grains. This should be investigated using a larger sample however. The molecular abundance in a few DLA systems (and in particular in the two systems where H2 is detected) is consistent with what is seen in the Magellanic clouds. But most of the DLA measurements are well below these values. This is probably partly due to small amounts of dust and/or high UV flux. We argue however that the lack of molecules is a direct consequence of high kinetic temperature (T [FORMULA] 3000 K) implying a low formation rate of H2 onto dust grains. Therefore, most of the DLA systems arise in warm and diffuse neutral gas.

Key words: galaxies: evolution – galaxies: formation – galaxies: ISM – galaxies: quasars: absorption lines – cosmology: observations

* Based on observations collected during ESO programmes 65.P-0038 and ESO 65.O-0063 at the European Southern Observatory with UVES on the 8.2m KUEYEN telescope operated on Cerro Paranal, Chile

Send offprint requests to: P. Petitjean (petitjean@iap.fr)

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© European Southern Observatory (ESO) 2000

Online publication: December 15, 2000