EDP Sciences
Free access
Volume 415, Number 3, March I 2004
Page(s) 879 - 884
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20034584

A&A 415, 879-884 (2004)
DOI: 10.1051/0004-6361:20034584

Hints of star formation at z $\mathsf{>6}$: The chemical abundances of the DLA system in the QSO BRI 1202-0725 (z $\mathsf{_{abs}=4.383}$)

V. D'Odorico and P. Molaro

INAF - Osservatorio Astronomico di Trieste, via G.B. Tiepolo, 11, 34131 Trieste, Italy
(Received 23 July 2003 / Accepted 30 October 2003 )

The Damped Ly- $\alpha$ (DLA) absorber at redshift $z_{\rm abs}=4.383$ observed toward QSO BRI 1202-0725 is studied by means of high resolution (FWHM  $\approx 7$ km s -1) VLT-UVES spectra. We refine a previously determined Si abundance and derive with confidence abundances for C, N and O which are poorly known in DLAs. The [O/Fe] ratio is ~ 0.6, but we cannot establish if iron is partially locked into dust grains. The [C/Fe]  $=0.08\pm
0.13$ and [Si/C]  $=0.31\pm0.07$. [N/O] and [N/Si] are about -1, which is consistent with the majority of DLAs. This value is much larger than the one observed for the DLA toward QSO J 0307-4945 at $z_{\rm abs} = 4.466$. The current interpretation of the bimodal distribution of N abundances in DLAs implies that large [ N/ $\alpha$]  values correspond to relatively old systems. Adopting a scale time of 500 Myrs for full N production by intermediate mass stars, the onset of star formation in our DLA took place already at redshift > 6.

Key words: galaxies: abundances -- galaxies: high-redshift -- galaxies: quasars: absorption lines -- cosmology: observations

Offprint request: V. D'Odorico, dodorico@sissa.it

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