The sub - damped Ly α system toward HE 0001 − 2340 : galaxy formation at z ≈ 2 ⋆

We present a detailed analysis of chemical abundances in a sub-damped Ly α absorber (sub-DLA) at z = 2.187 towards the quasar HE 0001−2340 (zem = 2.28). Our study is based on high-resolution (R ≈ 45, 000) spectroscopic data from the UVES instrument installed on the ESO Very Large Telescope (VLT). This sub-DLA system consists of at least 25 individual subcomponents spanning a restframe velocity range of ∼ 400 km s −1. For the dominating neutral gas component at v rel = +13 km s −1 (relative to z = 2.187) we derive an oxygen abundance of [O/H]= −1.81 ± 0.07 (1/65 solar). With its extremely low nitrogen content ([N/H] −3.3 and [N/O] −1.5) the absorber exhibits a classic massive star abundance pattern. Our measurements place the z = 2.187 absorber towards HE 0001−2340 among the systems with the lowest ever measured [N/α] ratios in the Universe. The low [N/O] value is consistent with the idea that primary nitrogen production by the very first stars have enriched the intergalactic gas to a level of [N/O]≈ −1.5. Peculiar abundances are found in the outermost blue components near −350 km s −1 (in the z = 2.187 restframe) where we observe significant overabundances of phosphorus ([P/C]≈ +1.5), silicon ([Si/C]≈ +0.6), and aluminum ([Al/C]≈ +0.5) after correcting for the effects of ionization. Our study suggests that the sight line passes through the gaseous environment of one or more stellar clusters that locally enriched their interstellar neighbourhood by supernova ejecta generating the observed abundance anomalies. The large velocity spread of the entire absorption system points to a merger that triggers the formation of these clusters. We thus may be observing a young galaxy at z ≈ 2 that currently forms out of a merger event.