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

Abstract

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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Figure 1: Locations of the observed stars in Galactocentric Cartesian coordinates.
Figure 2: Chemical abundances of the observed stars.
Figure 3: Comparison of the positions of the observed stars with an N-body simulation.

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Acknowledgements

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.

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Authors and Affiliations

Authors

Contributions

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.

Corresponding author

Correspondence to Maria Bergemann.

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Extended data figures and tables

Extended Data Figure 1 Chemical abundances of the observed stars.

a, b, Chemical abundance ratios [Mg/Fe] (a) and [Ti/Fe] (b), plotted against metallicity ([Fe/H]), in the TriAnd and A13 overdensities, as well as in Milky Way disk and halo stars; the Fornax and Sagittarius (Sgr) dwarf spheroidal galaxies (dSph); open clusters in the Galactic outer disk; and globular clusters (with error bars reflecting intracluster abundance variation, derived as the r.m.s. variance of the sample, with N = 13 for the M3 cluster and N = 25 for M71). Source data are provided in Extended Data Tables 1 and 3.

Source data

Extended Data Figure 2 Comparison of the observed spectrum of elemental abundances and a model spectrum for a star in the A13 overdensity.

Shown are the Keck spectrum of the star 2MASS 07154242+6704006 (grey dots) and the best-fit model spectrum (red line). We used the Na i lines at 6,154 Å and 6,160 Å to determine the Na abundance of the star.

Source data

Extended Data Figure 3 Line-of-sight velocities (vlos) of the observed stars, plotted against Galactic longitude (l).

GSR, Galactic standard of rest. See Methods for further details.

Extended Data Table 1 Coordinates and 2MASS photometric magnitudes of the observed stars
Extended Data Table 2 Radial velocities, stellar parameters and distances of the observed stars
Extended Data Table 3 Chemical abundances of oxygen, magnesium, sodium, titanium, europium and barium in the observed stars

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Bergemann, M., Sesar, B., Cohen, J. et al. Two chemically similar stellar overdensities on opposite sides of the plane of the Galactic disk. Nature 555, 334–337 (2018). https://doi.org/10.1038/nature25490

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