Kinematics and star formation activity in the z=2.03954 damped Lyman-alpha system towards PKS 0458-020

Abstract

We present UVES observations of the log N(H ) = 21.7 damped Lyman-α system at zabs = 2.03954 towards the quasar PKS 0458−020. H Lyman-α emission is detected in the center of the damped Lyman-α absorption trough. Metallicities are derived for Mg , Si , P , Cr , Mn , Fe  and Zn  and are found to be −1.21 ± 0.12, −1.28 ± 0.20, −1.54 ± 0.11, −1.66 ± 0.10, −2.05 ± 0.11, −1.87 ± 0.11, −1.22 ± 0.10, respectively, relative to solar. The depletion factor is therefore of the order of [Zn/Fe] = 0.65. We observe metal absorption lines to be blueshifted compared to the Lyman-α emission up to a maximum of ∼100 and 200 km s−1 for low and high-ionization species respectively. This can be interpreted either as the consequence of rotation in a large (∼7 kpc) disk or as the imprint of a galactic wind. The star formation rate (SFR) derived from the Lyman-α emission, 1.6M⊙yr −1 , is compared with that estimated from the observed C  ∗ absorption. No molecular hydrogen is detected in our data, yielding a molecular fraction log f < −6.52. This absence of H2 can be explained as the consequence of a high ambient UV flux which is one order of magnitude larger than the radiation field in the ISM of our Galaxy and originates in the observed emitting region.