The most metal-poor damped Lyα systems: insights into chemical evolution in the very metal-poor regime

Creators: Cooke, Ryan; Pettini, Max; Steidel, Charles C.; Rudie, Gwen C.; Nissen, Poul E.

Abstract

We present a high spectral resolution survey of the most metal-poor damped Lyα absorption systems (DLAs) aimed at probing the nature and nucleosynthesis of the earliest generations of stars. Our survey comprises 22 systems with iron abundance less than 1/100 solar; observations of seven of these are reported here for the first time. Together with recent measures of the abundances of C and O in Galactic metal-poor stars, we reinvestigate the trend of C/O in the very metal-poor (VMP) regime and we compare, for the first time, the O/Fe ratios in the most metal-poor DLAs and in halo stars. We confirm the near-solar values of C/O in DLAs at the lowest metallicities probed, and find that their distribution is in agreement with that seen in Galactic halo stars. We find that the O/Fe ratio in VMP DLAs is essentially constant, and shows very little dispersion, with a mean [〈O/Fe〉]=+0.39 ± 0.12, in good agreement with the values measured in Galactic halo stars when the oxygen abundance is measured from the [O i] λ6300 line. We speculate that such good agreement in the observed abundance trends points to a universal origin for these metals. In view of this agreement, we construct the abundance pattern for a typical VMP DLA and compare it to model calculations of Population II and Population III nucleosynthesis to determine the origin of the metals in VMP DLAs. Our results suggest that the most metal-poor DLAs may have been enriched by a generation of metal-free stars; however, given that abundance measurements are currently available for only a few elements, we cannot yet rule out an additional contribution from Population II stars.

Additional Information

© 2011 The Authors. Monthly Notices of the Royal Astronomical Society © 2011 RAS. Accepted 2011 July 1. Received 2011 July 1; in original form 2011 April 4. Article first published online: 24 Aug 2011. Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile [VLT programme IDs 67.A-0078(A), 69.A-0613(A), 083.A-0042(A), 085.A-0109(A)], and at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. Keck telescope time was granted by NOAO, through the Telescope System Instrumentation Program (TSIP). TSIP is funded by NSF. We are grateful to the relevant time assignment committees for their continuing support of this demanding observational programme, and to the staff astronomers at the VLT and Keck Observatories for their competent assistance with the observations. We also thank an anonymous referee who provided valuable comments that improved the presentation of this work. Tom Barlow and Michael Murphy generously shared their echelle data reduction software. Valuable advice and help with various aspects of the work described in this paper was provided by Bob Carswell, Paul Hewett and Regina Jorgenson. We thank the Hawaiian people for the opportunity to observe from Mauna Kea; without their hospitality, this work would not have been possible. RC is jointly funded by the Cambridge Overseas Trust and the Cambridge Commonwealth/Australia Trust with an Allen Cambridge Australia Trust Scholarship. CCS's research is partly supported by grants AST-0606912 and AST-0908805 from the US National Science Foundation.

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