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Issue A&A
Volume 385, Number 3, April III 2002
Page(s) 778 - 792
Section Extragalactic astronomy
DOI 10.1051/0004-6361:20020181



A&A 385, 778-792 (2002)
DOI: 10.1051/0004-6361:20020181

Metal abundances and ionization conditions in a possibly dust-free damped Ly $\alpha$ system at $\mathsf{\vec{z}=2.3}$

S. Lopez1, D. Reimers2, S. D'Odorico3 and J. X. Prochaska4

1  Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
2  Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
3  ESO, Karl-Schwarzschildstr. 1, Garching b. Muenchen, Germany
4  The Observatories of the Carnegie Institute of Washington, 813 Santa Barbara St., Pasadena, CA 91101, USA

(Received 10 October 2001 / Accepted 28 January 2002)

Abstract
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 $\ion$, $\ion$, $\ion{ii}$, $\ion{ii}$, $\ion{P}{ii}$, $\ion{S}{ii}$, $\ion{i}$, $\ion{ii}$, $\ion{ii}$, $\ion{ii}$, $\ion{ii}$, $\ion{C}{ii}$ $\ion{O}{vi}$ $\lambda1032$, and $\ion{iii}$ and put upper limits on the abundances of $\ion{ii}$, $\ion{ii}$, $\ion{iii}$, $\ion{P}{iii}$, and $\ion{N}{ii}$. The metallicity of this system is 1/12 solar at a neutral hydrogen column density of $\log N(\ion)=20.7$. From the observed ratios [Zn/Cr] $=-0.01\pm0.05$ and [S/Si] $=-0.06\pm0.03$ 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 $\ion{ii}$ extending over 2.5 orders of magnitude in $N(\ion{ii})$. 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 $\ion{iii}$/low-ion ratios suggest the mildly ionized gas occurs in shells surrounding neutral cores of $\ion{ii}$.


Key words: cosmology: observations -- quasars: individual: HE 2243-6031 -- quasars: general -- quasars: absorption lines

Offprint request: S. Lopez, slopez@das.uchile.cl

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