Two chemically similar stellar overdensities on opposite sides of the plane of the Galactic disk

  • Nature volume 555, pages 334337 (15 March 2018)
  • doi:10.1038/nature25490
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Our Galaxy is thought to have an active evolutionary history, dominated over the past ten billion years or so by star formation, the accretion of cold gas and, in particular, the merging of clumps of baryonic and dark matter1,2. The stellar halo—the faint, roughly spherical component of the Galaxy—reveals rich ‘fossil’ evidence of these interactions, in the form of stellar streams, substructures and chemically distinct stellar components3,4,5. The effects of interactions with dwarf galaxies on the content and morphology of the Galactic disk are still being explored. Recent studies have identified kinematically distinct stellar substructures and moving groups of stars in our Galaxy, which may have extragalactic origins6,7. There is also mounting evidence that stellar overdensities (regions with greater-than-average stellar density) at the interface between the outer disk and the halo could have been caused by the interaction of a dwarf galaxy with the disk8,9,10. Here we report a spectroscopic analysis of 14 stars from two stellar overdensities, each lying about five kiloparsecs above or below the Galactic plane—locations suggestive of an association with the stellar halo. We find that the chemical compositions of these two groups of stars are almost identical, both within and between these overdensities, and closely match the abundance patterns of stars in the Galactic disk. We conclude that these stars came from the disk, and that the overdensities that they are part of were created by tidal interactions of the disk with passing or merging dwarf galaxies11,12.

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We thank I. Georgiev for help with the telluric correction of the stellar spectrum taken with the UVES spectrograph at the VLT. A.M.S. was supported by grants ESP2015-66134-R and ESP2017-82674-R (MINECO). K.V.J.’s contributions were supported by a grant from the National Science Foundation (AST-1614743). L.C. acknowledges support from the Australian Research Council (grants DP150100250 and FT160100402). Parts of this research were conducted by the Australian Research Council Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), through project number CE170100013. M.B. acknowledges support from Collaborative Research Center SFB 881 (Heidelberg University, subproject A5) of the Deutsche Forschungsgemeinschaft. C.F.P.L. is supported by a Junior Fellow of the Simons Society of Fellows award from the Simons Foundation. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number OCI-1053575. R.S. is supported by a Royal Society University Research Fellowship. We thank S. Majewski and K. Cunha for interesting discussions on the topic, and J. Bovy for help with implementing the disk-flare profile. We thank T. Müller for assistance with the final, production-quality versions of all figures. We thank the people who realized the Keck Telescope and its instruments and those who operate and maintain the Keck Observatory. We thank the indigenous Hawaiian community for their generous hospitality on their sacred mountain.

Author information


  1. Max Planck Institute for Astronomy, Koenigstuhl 17, 69117 Heidelberg, Germany

    • Maria Bergemann
    •  & Andrew Gould
  2. Deutsche Börse AG, Mergenthalerallee 61, 65760 Eschborn, Germany

    • Branimir Sesar
  3. Division of Physics, Mathematics, and Astronomy, California Institute of Technology, Pasadena, California 91125, USA

    • Judith G. Cohen
  4. Institute of Space Sciences (ICE, CSIC), Carrer de Can Magrans, E-08193 Barcelona, Spain

    • Aldo M. Serenelli
  5. Institut d’Estudis Espacials de Catalunya (IEEC), C/Gran Capita, 2-4, E-08034 Barcelona, Spain

    • Aldo M. Serenelli
  6. Department of Natural Sciences, LaGuardia Community College, City University of New York, 31-10 Thomson Avenue, Long Island City, New York 11101, USA

    • Allyson Sheffield
  7. Fermi National Accelerator Laboratory, PO Box 500, Batavia, Illinois 60510, USA

    • Ting S. Li
  8. Research School of Astronomy and Astrophysics, Mount Stromlo Observatory, The Australian National University, Canberra, Australian Capital Territory 2611, Australia

    • Luca Casagrande
  9. ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia

    • Luca Casagrande
  10. Department of Astronomy, Columbia University, 550 West 120th Street, Mail Code 5246, New York, New York 10027, USA

    • Kathryn V. Johnston
    •  & Chervin F. P. Laporte
  11. Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, New Jersey 08544, USA

    • Adrian M. Price-Whelan
  12. Rudolf-Peierls Centre for Theoretical Physics, University of Oxford, 1 Keble Road, Oxford OX1 3NP, UK

    • Ralph Schönrich
  13. Korea Astronomy and Space Science Institute, Daejon 34055, South Korea

    • Andrew Gould
  14. Department of Astronomy, Ohio State University, 140 West 18th Avenue, Columbus, Ohio 43210, USA

    • Andrew Gould


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This project was led by M.B. and initiated by B.S. and K.V.J. The photometric selection of targets was made by B.S., T.S.L. and A.S. The high-resolution Keck spectra were obtained by J.G.C. The VLT proposal to observe the TriAnd star was prepared by B.S. Spectroscopic analysis of the spectra, including stellar parameters and chemical abundances, was carried out by M.B. L.C. measured stellar effective temperatures and A.M.S. carried out the Bayesian analysis of distances. C.F.P.L. provided the N-body simulation that describes the interaction of the Sagittarius galaxy with the Galactic disk. R.S. performed the analysis of stellar kinematics. A.M.P.-W. helped with interpretation of the results and comparison with the models. The manuscript was written mainly by M.B. and A.G. All authors contributed to the text and provided comments.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Maria Bergemann.

Reviewer Information Nature thanks R. Ibata and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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