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Linking dwarf galaxies to halo building blocks with the most metal-poor star in Sculptor

Abstract

Current cosmological models1,2 indicate that the Milky Way’s stellar halo was assembled from many smaller systems. On the basis of the apparent absence of the most metal-poor stars in present-day dwarf galaxies, recent studies3 claimed that the true Galactic building blocks must have been vastly different from the surviving dwarfs. The discovery of an extremely iron-poor star (S1020549) in the Sculptor dwarf galaxy based on a medium-resolution spectrum4 cast some doubt on this conclusion. Verification of the iron-deficiency, however, and measurements of additional elements, such as the α-element Mg, are necessary to demonstrate that the same type of stars produced the metals found in dwarf galaxies and the Galactic halo. Only then can dwarf galaxy stars be conclusively linked to early stellar halo assembly. Here we report high-resolution spectroscopic abundances for 11 elements in S1020549, confirming its iron abundance of less than 1/4,000th that of the Sun, and showing that the overall abundance pattern follows that seen in low-metallicity halo stars, including the α-elements. Such chemical similarity indicates that the systems destroyed to form the halo billions of years ago were not fundamentally different from the progenitors of present-day dwarfs, and suggests that the early chemical enrichment of all galaxies may be nearly identical.

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Figure 1: Spectral comparison of S1020549 with two other metal-poor stars.
Figure 2: Abundance ratios as a function of iron abundance in S1020549 and other metal-poor stars.

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Acknowledgements

Our data was gathered using the 6.5-m Magellan telescopes located at Las Campanas Observatory, Chile. We thank L. Hernquist for discussions on galaxy formation. A.F. acknowledges support through a Clay Fellowship administered by the Smithsonian Astrophysical Observatory. Support for this work was provided by NASA through a Hubble fellowship grant awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA (to E.N.K., who is a Hubble Fellow). J.D.S. acknowledges the support of a Vera Rubin Fellowship provided by the Carnegie Institution of Washington.

Author Contributions A.F. took the high-resolution observations, and led the analysis; E.N.K. provided the target; J.D.S. contributed to the analysis. All authors contributed to the writing of the paper.

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Correspondence to Anna Frebel.

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Frebel, A., Kirby, E. & Simon, J. Linking dwarf galaxies to halo building blocks with the most metal-poor star in Sculptor. Nature 464, 72–75 (2010). https://doi.org/10.1038/nature08772

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