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

¹ Institut d'Astrophysique de Paris, UMR7095 CNRS, Université Pierre & Marie Curie, 98bis boulevard Arago, 75014 Paris, France
² Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr. 12-14, 69120 Heidelberg, Germany e-mail: janine@ari.uni-heidelberg.de
³ Astrophysik, Universität Potsdam, Am Neuen Palais 10, 14469 Potsdam, Germany
⁴ LERMA, Observatoire de Paris, 61 avenue de l'Observatoire, 75014 Paris, France
⁵ European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Vitacura, Santiago, Chile
⁶ IUCAA, Post Bag 4, Ganesh Khind, Pune 411 007, India

Received: 25 July 2005
Accepted: 16 November 2005

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-α 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 , , , , , , , 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^-1, is compared with that estimated from the observed C ii^* absorption. No molecular hydrogen is detected in our data, yielding a molecular fraction  < -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.