Detection of molecular hydrogen in a near Solar-metallicity damped Lyman-alpha system at z_abs~2 toward Q 0551-366

Abstract

We report the detection of H_2, CI, CI*, CI** and Cl I lines in a near Solar-metallicity ([Zn/H]=-0.13) damped Lyman-alpha (DLA) system at z_abs=1.962 observed on the line of sight to the quasar Q 0551-366. The iron-peak elements, X=Fe, Cr and Mn are depleted compared to zinc, [X/Zn] -0.8, probably because they are tied up onto dust grains. Among the three detected H_2-bearing clouds, spanning 55 km s^-1 in velocity space, we derive a total molecular hydrogen column density N(H_2)=2.6x10^17 cm^-2 and a mean molecular fraction f=2N(H_2)/(2N(H_2)+N(HI))=1.7x10^-3. 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 CI-detected components independently of the presence or absence of H_2. The gas clouds in which H_2 is detected always have large densities, n_H>30 cm^-3, and low temperatures, T_01<~100 K. This shows that presence of dust, high particle density and/or low temperature are required for molecules to be present. The photo-dissociation rate derived in the components where H_2 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 H_2-bearing clouds.