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(HI)= 21.7 damped Lyman-alpha system at z=2.03954 towards the quasar PKS 0458-020. HI Lyman-alpha emission is detected in the center of the damped Lyman-alpha absorption trough. Metallicities are derived for MgII, SiII, PII, CrII, MnII, FeII and ZnII and are found to be -1.210.12, -1.280.20, -1.540.11, -1.660.10, -2.050.11, -1.870.11, -1.220.10, 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-alpha emission up to a maximum of 100 and 200 km/s for low and high-ionization species respectively. This can be interpreted either as the consequence of rotation in a large (~7kpc) disk or as the imprint of a galactic wind. The star formation rate (SFR) derived from the Lyman-alpha emission, 1.6 solar masses/yr, is compared with that estimated from the observed CII* absorption. No molecular hydrogen is detected in our data, yielding a molecular fraction 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.
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