The most metal-poor damped Lyman α system at z <3: constraints on early nucleosynthesis

International audienceTo constrain the conditions for very early nucleosynthesis in the Universe we compare the chemical enrichment pattern of an extremely metal-poor damped Lyman α (DLA) absorber with predictions from recent explosive nucleosynthesis model calculations. For this, we have analyzed chemical abundances in the DLA system at zabs=2.6183 toward the quasar Q0913+072 (zem=2.785) using public UVES/VLT high spectral resolution data. The total neutral hydrogen column density in this absorber is log N(H i) = 20.36 ± 0.05. Accurate column densities are derived for C ii, N i, O i, Al ii, Si ii, and Fe ii. Upper limits are given for Fe iii and Ni ii. With [{C/H}]=-2.83 ± 0.05, [{N/H}]=-3.84 ± 0.11, and [{O/H}]=-2.47 ± 0.05, this system represents one of the most metal-poor DLA systems investigated so far. It offers the unique opportunity to measure accurate CNO abundances in a protogalactic structure at high redshift. Given the very low overall abundance level and the observed abundance pattern, the data suggest that the chemical evolution of this DLA system is dominated by one or at most a few stellar generations. With reference to numerical model calculations, the chemical abundances in the DLA system are consistent with an enrichment from a single starburst of a zero-metallicity population of massive stars ( 10{-}50 M_⊙) exploding as core-collapse Supernovae (SNe), i.e., the classical type II Supernovae (SNe II), and possibly as hyper-energetic (E>1051 erg) core-collapse Supernovae, so-called Hypernovae (HNe), as well. In contrast, models using non-zero metallicity progenitors or other explosion mechanisms, such as pair-instability Supernovae (PISNe) or type Ia Supernovae (SNe Ia), do not match the observed abundance pattern. Comparing our results with recent estimates for the global chemical evolution of the intergalactic medium (IGM) and early galactic structures shows that the observed metal abundances in the DLA system toward Q0913+072 are only slightly above the level expected for the intergalactic medium (IGM) at z≈2.6, but significantly lower than what is expected for the interstellar medium (ISM) in galaxies at that redshift. This implies that this DLA system has recently condensed out of the IGM and that local star formation in this protogalaxy has not yet contributed significantly to the metal budget in the gas