Hints of star formation at z > 6 : the chemical abundances of the DLA system in the QSO BRI 1202 - 0725 ( z abs = 4 . 383 ) ⋆

The Damped Ly-α (DLA) absorber at redshift zabs = 4.383 observed toward QSO BRI 1202-0725 is studied by means of high resolution (FWHM ≈ 7 km s) 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± 0.13 and [Si/C] = 0.31 ± 0.07. [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 zabs = 4.466. 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.