We have obtained a high resolution , high S/N UVES spectrum of the bright QSO HE 2243 - 6031 to analyze the damped Ly \alpha system ( DLA ) observed at z = 2.33 .
We measure column densities for H i , N i , Al ii , Si ii , P ii , S ii , Ar i , Cr ii , Fe ii , Ni ii , Zn ii , C ii O i , and Al iii ; and put upper limits on the abundances of Mn ii , S iii , Fe iii , P iii , and N ii . The metallicity of this system is 1 / 12 solar at a neutral hydrogen column density of \log N ( { H \textsc { i } } ) = 20.7 .
From the observed ratios [ Zn/Cr ] = -0.01 \pm 0.05 and [ S/Si ] = -0.06 \pm 0.03 we conclude that dust is very likely absent from the ISM of this protogalaxy .
We observe an enhancement of the \alpha /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 Fe ii extending over 2.5 orders of magnitude in N ( { Fe \textsc { ii } } ) .
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 Al iii /low-ion ratios suggest the mildly ionized gas occurs in shells surrounding neutral cores of Al ii .