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

Abstract

We present UVES observations of the log N(H I) = 21.7 damped Lyman-α system at Z_abs = 2.03954 towards the quasar PKS 0458-020. H I Lyman-α emission is detected in the center of the damped Lyman- absorption trough. Metallicities are derived for Mg II, Si II, P II, Cr II, Mn II, Fe II and Zn II and are found to be -1.21 ± 0.12, -1.28 ± 0.20, -1.54 ± 0.11, -1.66 ± 0.19, -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, 1.6 yr MΘ ^-1, is compared with that estimated from the observed C II absorption. No molecular hydrogen is detected in our data, yielding a molecular fraction log ƒ < -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.