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Extremely metal-poor stars in dwarf galaxies

Published online by Cambridge University Press:  09 March 2010

Anna Frebel ,
Joshua D. Simon ,
Evan Kirby ,
Marla Geha  and
Beth Willman
Anna Frebel
Affiliation:
Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA email: afrebel@cfa.harvard.edu
Joshua D. Simon
Affiliation:
Observatories of the Carnegie Institution of Washington, Pasadena, CA 91101, USA email: jsimon@ociw.edu
Evan Kirby
Affiliation:
California Institute of Technology, Pasadena, CA 91106, USA email enk@astro.caltech.edu
Marla Geha
Affiliation:
Astronomy Department, Yale University, New Haven, CT 06520, USA email marla.geha@yale.edu
Beth Willman
Affiliation:
Haverford College, Haverford, PA 19041, USA email: bwillman@haverford.edu
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Abstract

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We present Keck/HIRES spectra of six metal-poor stars in two of the ultra-faint dwarf galaxies orbiting the Milky Way, Ursa Major II and Coma Berenices, and a Magellan/MIKE spectrum of a star in the classical dwarf spheroidal galaxy (dSph) Sculptor. Our data include the first high-resolution spectroscopic observations of extremely metal-poor stars ([Fe/H] < −3.0) not belonging to the Milky Way (MW) stellar halo field population. We obtain abundance measurements and upper limits for up to 26 elements between carbon and europium. The stars span a range of −3.8 < [Fe/H] < −2.3, with the ultra-faints having large spreads in Fe. A comparison with MW halo stars of similar metallicity reveals substantial agreement between the abundance patterns of the ultra-faint dwarf galaxies and Sculptor and the MW halo for the light, α and iron-peak elements (C to Zn). This agreement contrasts with the results of earlier studies of more metal-rich stars (−2.5 ≲[Fe/H]≲ −1.0) in more luminous dwarfs, which found significant abundance discrepancies with respect to the MW halo data. The abundances of neutron-capture elements (Sr to Eu) in all three galaxies are extremely low, consistent with the most metal-poor halo stars, but not with the typical halo abundance pattern at [Fe/H]≳ −3.0. Our results are broadly consistent with a galaxy formation model which predicts that massive dwarf galaxies are the source of the metal-rich component ([Fe/H]≳ −2.5) of the MW inner halo, but we propose that dwarf galaxies similar to the dSphs are the primary contributors to the metal-poor end of the metallicity distribution of the MW outer halo.

Keywords

stars: abundancesstars: Population IIGalaxy: formationGalaxy: halogalaxies: dwarf(galaxies:) Local Groupgalaxies: stellar content(cosmology:) early universe
Type
Contributed Papers
Information
Proceedings of the International Astronomical Union , Volume 5 , Symposium S265: Chemical Abundances in the Universe: Connecting First Stars to Planets , August 2009 , pp. 237 - 240
Copyright
Copyright © International Astronomical Union 2010

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