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Published March 15, 2018 | Accepted Version
Journal Article Open

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


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 matter. 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 components. 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 origins. 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 disk. 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 galaxies.

Additional Information

© 2018 Macmillan. Received: 18 August 2017. Accepted: 11 December 2017. Published online: 26 February 2018. 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. The authors declare no competing financial interests.

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