Hints of star formation at z : The chemical abundances of the DLA system in the QSO BRI 1202-0725 (z)^*

INAF - Osservatorio Astronomico di Trieste, via G.B. Tiepolo, 11, 34131 Trieste, Italy

Corresponding author: V. D'Odorico, dodorico@sissa.it

Received: 23 July 2003
Accepted: 30 October 2003

Abstract

The Damped Ly-α (DLA) absorber at redshift observed toward QSO BRI 1202-0725 is studied by means of high resolution (FWHM  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]  and [Si/C] . [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 . The current interpretation of the bimodal distribution of N abundances in DLAs implies that large [ N/α]  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.