The sub-DLA toward HE0001-2340: galaxy formation at z=2

Abstract

We present a detailed analysis of chemical abundances in a sub-damped Ly alpha absorber (sub-DLA) at z=2.187 towards the quasar HE0001-2340 (z_em=2.28) using VLT/UVES data. This sub-DLA system consists of at least 25 individual subcomponents spanning a restframe velocity range of ~400 km/s. Detected species include CII, CIV, NI, NII, OI, MgII, AlII, AlIII, SiII, SiIV, PII, FeII, and possibly DI. For the dominating neutral gas component at v=+13 km/s we derive an oxygen abundance of [O/H]=-1.81. With its extremely low nitrogen content ([N/H]<-3.3 and [N/O]<-1.5) the absorber exhibits a classic massive star abundance pattern. 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, where we observe significant overabundances of phosphorus ([P/C] ~ +1.5), silicon ([Si/C] ~ +0.6), and aluminum ([Al/C] ~ +0.5). 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 (abstract truncated).