Excitation mechanisms in newly discovered H₂-bearing damped Lyman-α clouds: systems with low molecular fractions ^*,**

¹ European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Vitacura, Santiago, Chile e-mail: pnoterda@eso.org
² Institut d'Astrophysique de Paris, CNRS – Université Pierre et Marie Curie, 98bis Boulevard Arago, 75014 Paris, France
³ LERMA, Observatoire de Paris, 61 Avenue de l'Observatoire, 75014 Paris, France
⁴ LUTH, Observatoire de Paris, 61 Avenue de l'Observatoire, 75014 Paris, France
⁵ IUCAA, Post Bag 4, Ganesh Khind, Pune 411 007, India

Received: 5 June 2007
Accepted: 20 July 2007

Abstract

Aims. We probe the physical conditions in high-redshift damped Lyman-α systems (DLAs) using the observed molecular fraction and the rotational excitation of molecular hydrogen.


Methods.We search for Lyman- and Werner-band absorption lines of molecular hydrogen in the VLT/UVES spectra of background QSOs at the redshift of known DLAs.

Results.We report two new detections of molecular hydrogen in the systems at and 1.989 toward, respectively, HE 0027-1836 and HE 2318-1107, discovered in the course of the Hamburg-ESO DLA survey. We also present a detailed analysis of our recent H₂ detection toward Q 2343+125. All three systems have low molecular fractions, , with HHH i. Only one such H₂ system was known previously. Two of them (toward Q 2343+125 and HE 2318-1107) have high-metallicities, [ X/H, whereas the DLA toward HE 0027-1836 is the system with the lowest metallicity ([ Zn/H) among known H₂-bearing DLAs. The depletion patterns for Si, S, Ti, Cr, Mn, Fe and Ni in the three systems are found to be very similar to what is observed in diffuse gas of the Galactic halo. Molecular hydrogen absorption from rotational levels up to is observed in a single well-defined component toward HE 0027-1836. We show that the width (Doppler parameter) of the H₂ lines increases with increasing J and that the kinetic energy derived from the Doppler parameter is linearly dependent on the relative energy of the rotational levels. There is however no velocity shift between lines from different rotational levels. The excitation temperature is found to be 90 K for to and ~500 K for higher J levels.
Single isothermal PDR models fail to reproduce the observed rotational excitations. A two-component model is needed: one component of low density (~50 cm^-3) with weak illumination (χ = 1) to explain the rotational levels and another of high density (~500 cm^-3) with strong illumination () for  levels. However, the juxtaposition of these two PDR components may be ad-hoc and the multicomponent structure could result either from turbulent dissipation or C-shocks.