Metal abundances and ionization conditions in a possibly dust-free damped Lyα system at z = 2.3

Abstract

We have obtained a high resolution, high S/N UVES spectrum of the bright QSO HE 2243-6031 to analyze the damped Lyα system (DLA) observed at z = 2.33. We measure column densities for HI, NI, AlII, SiII, PII, SII, ArI, CrII, FeII, NiII, ZnII, CII, OI, and AlIII and put upper limits on the abundances of MnII, SIII, FeIII, PIII, and NII. The metallicity of this system is 1/12 solar at a neutral hydrogen column density of log N(HI) = 20.7. From the observed ratios [Zn/Cr] = -0.01 ± 0.05 and [S/Si] = -0.06 ± 0.03 we conclude that dust is very likely absent from the ISM of this protogalaxy. We observe an enhancement of the α/Fe-peak ratios of +0.2 dex for various elements, a marked odd-even effect in Mn, and a strong underabundance of N relative to Si and S, [N/Si,S] = -1 at [Si/H] = -0.86. All of these ratios support an environment that is in an early evolutionary stage, where the onset of star formation has begun only shortly before the DLA was observed. We also perform a cloud-by-cloud analysis - without precedent at high redshift - and find a tight correlation of all low-ionization species with respect to FeII extending over 2.5 orders of magnitude in N(FeII). We interpret this trend as being due to homogeneous physical conditions (very mild ionization effects, common dust-destruction histories, same chemical composition) and propose that this line-of-sight encounters absorbing clouds that share a common environment. In addition, photoionization models show that these single clouds are shielded from the external ionizing radiation, so the fraction of ionized gas is small and, except for argon, does not influence the measured metal abundances. The observed AlIII/low-ion ratios suggest the mildly ionized gas occurs in shells surrounding neutral cores of AlII.