Detection of molecular hydrogen in a near Solar-metallicity damped Lyman-α system at ≈ 2 toward Q 0551-366^*

¹ European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Vitacura, Santiago, Chile
² IUCAA, Post Bag 4, Ganesh Khind, Pune 411 007, India e-mail: anand@iucaa.ernet.in
³ Institut d'Astrophysique de Paris – CNRS, 98bis boulevard Arago, 75014 Paris, France e-mail: petitjean@iap.fr
⁴ LERMA, Observatoire de Paris, 61 avenue de l'Observatoire, 75014, Paris, France

Corresponding author: C. Ledoux, cledoux@eso.org

Received: 11 May 2001
Accepted: 24 June 2002

Abstract

We report the detection of H₂, C i, C i  , C i  and Cl i lines in a near Solar-metallicity ([ Zn/H) damped Lyman-α (DLA) system at observed on the line of sight to the quasar Q 0551-366. The iron-peak elements, , Cr and Mn are depleted compared to zinc, [ X/Zn, probably because they are tied up onto dust grains. Among the three detected H₂-bearing clouds, spanning 55 km s^-1 in velocity space, we derive a total molecular hydrogen column density H cm^-2 and a mean molecular fraction HHH i . The depletion of heavy elements (S, Si, Mg, Mn, Cr, Fe, Ni and Ti) in the central component is similar to that observed in the diffuse neutral gas of the Galactic halo. This depletion is approximately the same in the six C i-detected components independently of the presence or absence of H₂. The gas clouds in which H₂ is detected always have large densities, cm^-3, and low temperatures, K. This shows that presence of dust, high particle density and/or low temperature is required for molecules to be present. The photo-dissociation rate derived in the components where H₂ is detected suggests the existence of a local UV radiation field similar in strength to the one in the Galaxy. Star formation therefore probably occurs near these H₂-bearing clouds.