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Issue A&A
Volume 440, Number 3, September IV 2005
Page(s) 819 - 829
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20042064



A&A 440, 819-829 (2005)
DOI: 10.1051/0004-6361:20042064

The sub-damped Ly $\alpha$ system toward HE 0001-2340: galaxy formation at z $\approx$ 2

P. Richter1, C. Ledoux2, P. Petitjean3 and J. Bergeron3

1  Institut für Astrophysik und Extraterrestrische Forschung, Auf dem Hügel 71, 53121 Bonn, Germany
    e-mail: prichter@astro.uni-bonn.de
2  European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Vitacura, Santiago, Chile
3  Institut d'Astrophysique de Paris - CNRS, 98bis Boulevard Arago, 75014 Paris, France

(Received 24 September 2004 / Accepted 11 May 2005 )

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 HE 0001-2340 ( $z_{\rm em}=2.28$). Our study is based on high-resolution ( $R\approx 45\,000$) 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. The total neutral hydrogen column density is log N(H I $)\approx19.7$. Detected species include C II, C IV, N I, N II, O I, Mg II, Al II, Al III, Si II, Si IV, P II, Fe II, and possibly D I. For the dominating neutral gas component at $v_{\rm rel}=+13$ km s-1 (relative to z=2.187) we derive an oxygen abundance of $\rm [O/H]=-1.81\pm0.07$ (1/65 solar). With its extremely low nitrogen content ( $[{\rm N/H]}\la -3.3$ and $[{\rm N/O}]\lesssim -1.5$) 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/$\alpha$] ratios. 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 $\rm [N/O]\approx-1.5$. 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 ( ${\rm [P/C]}\approx+1.5$), silicon ( $[{\rm Si/C}]\approx+0.6$), and aluminum ( $[{\rm Al/C}]\approx+0.5$) after correcting for the effects of ionization. Our study suggests that the line of sight 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 $z\approx2$ that is forming out of a merger event.


Key words: cosmology: observations -- Galaxy: abundances -- Galaxy: evolution -- quasars: absorption lines

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© ESO 2005


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