Abstract
Gravitational interactions between the Large Magellanic Cloud (LMC) and the stellar and dark matter halo of the Milky Way are expected to give rise to disequilibrium phenomena in the outer Milky Way1,2,3,4,5,6,7. A local wake is predicted to trail the orbit of the LMC, and a large-scale overdensity is predicted to exist across a large area of the northern Galactic hemisphere. Here we report the detection of both the local wake and northern overdensity (hereafter the ‘collective response’) in a map of the Galaxy based on 1,301 stars at Galactocentric distances between 60 and 100 kiloparsecs. The location of the wake is in good agreement with an N-body simulation that includes the dynamical effect of the LMC on the Milky Way halo. The density contrast of the wake and collective response are stronger in the data than in the simulation. The detection of a strong local wake is independent evidence that the Magellanic clouds are on their first orbit around the Milky Way. The wake traces the path of the LMC, which will provide insight into the orbit of the LMC, which in turn is a sensitive probe of the mass of the LMC and the Milky Way. These data demonstrate that the outer halo is not in dynamical equilibrium, as is often assumed. The morphology and strength of the wake could be used to test the nature of dark matter and gravity.
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Data availability
The K giant catalogue used in this paper is available at https://doi.org/10.7910/DVN/2D1H8J.
Code availability
We have opted not to make the code used in this manuscript available because the data reduction and analysis is straightforward and can be easily reproduced following the methods described herein.
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Acknowledgements
C.C. is partially supported by the Packard Foundation. R.P.N. acknowledges an Ashford Fellowship and Peirce Fellowship granted by Harvard University. G.B. and N.G.-C. are supported by HST grant AR 15004, NASA ATP grant 17-ATP17-0006, NSF CAREER AST-1941096. A.B. acknowledges support from NASA through HST grant HST-GO-15930. All the simulations were run on El-Gato supercomputer, which was supported by the National Science Foundation under grant no. 1228509. We have made use of data from the European Space Agency mission Gaia (http://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC; see http://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, and NEOWISE, which is a project of the Jet Propulsion Laboratory/ California Institute of Technology. WISE and NEOWISE are funded by the National Aeronautics and Space Administration.
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C.C. and R.P.N. jointly conceived of the project. C.C. led the analysis of the data. N.G.-C. and G.B. provided the simulation data and aided in its interpretation. All authors contributed to aspects of the analysis and to the writing of the manuscript.
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Extended data figures and tables
Extended Data Fig. 1 Photometric selection of giants.
Colour–colour diagram for stars at high Galactic latitude (b > 45°) with Gaia parallax π < 0.1 mas. Two sequences are clearly visible, with the upper branch being associated with giant stars, and the lower with dwarf stars. The red lines mark the selection boundary used in this work.
Extended Data Fig. 2 Test of photometric distances.
Comparison of our photometric distances against literature values for satellite galaxies around the Milky Way (red) and globular clusters (blue). Error bars represent the 1σ scatter in the photometric distances and dotted lines mark ±20% about the one-to-one line. The dwarf galaxies include Fornax, Draco, Sculptor, Carina, Ursa Minor, and the LMC and the SMC, and span average metallicities from [Fe/H] ≈ −0.5 (LMC) to [Fe/H] ≈ −2.2 (Ursa Minor)42.
Extended Data Fig. 3 Maps of unsmoothed star counts.
Mollweide projection maps of the observed sample of giants with pixels of size 13.4 square degrees. a, Map with no masking of known structure, either in sky coordinates or proper motions. The two main features identified here, the transient wake and the collective response, are still clearly visible even in this unfiltered map. The LMC and the SMC appear in the lower right as a merged region of high density. Other features include the stellar disk + bulge in the centre, and the Sagittarius Stream both in the north-centre and lower left and right. b, Map showing the filtered catalogue used in Fig. 1a.
Extended Data Fig. 4 Proper motions of the halo sample.
a, Proper motions of the K giants at 60 < Rgal < 100 kpc with 12 < W1 < 15 and the LMC and the SMC removed via on-sky selection. b, Solar reflex-corrected proper motions. Notice the much tighter distribution of stars near (0, 0). In this panel, the Sagittarius dSph is visible at (−2.5, −0.7) and LMC and SMC stars not removed by the on-sky selection are visible as a narrow vertical strip at μα ≳ 0.5 mas yr−1. The northern Sagittarius arm is the large overdensity at (−0.7, 0.0) and a southern arm of Sagittarius is the diagonal cluster of points at μδ < −0.5 mas yr−1. c, Reflex-corrected proper motions focusing on the region of the sky containing the northern Sagittarius arm at l ≈ 0°. The red box indicates our selection for removing this feature. d, Stars not in the selection shown in c. Our selection for low proper motion stars is indicated by a blue circle in c and d.
Extended Data Fig. 5 An annotated map of the outer halo.
As in Extended Data Fig. 3 (right panel), now shown with the predicted orbit of the LMC (solid white line), a line at −25° declination (grey dashed line, marking the approximate limit of northern hemisphere surveys) and the locations of the two regions used to measure density ratios in Fig. 2 (solid yellow lines). The yellow region in the north measures the collective response while the yellow region in the south measures the local wake.
Extended Data Fig. 6 Predicted density distribution of a smooth model.
The model18 has a smooth (oblate) stellar halo. a, The unfiltered smooth model. b, The smooth model with the same selection criteria as used in the data, including various coordinate and proper motion cuts. Both maps have been smoothed by a Gaussian with FWHM = 30°. Unlike the data, this map shows no obvious structure.
Extended Data Fig. 7 RR Lyrae as a probe of the stellar halo.
a, Binned all-sky map of RR Lyrae stars identified in Pan-STARRS data51. The data are restricted to declinations greater than −30°. The wake is clearly visible in the lower left quadrant (compare with Extended Data Fig. 4). b, Measured densities in the wake and collective response regions for RR Lyrae (blue) compared with the K giants (black; compare with Fig. 2). Both the wake and collective response are clearly detected in the RR Lyrae.
Extended Data Fig. 8 Predicted response of the Galactic halo to the LMC.
The N-body simulation presented in Fig. 1b is shown here without any selections, either in proper motions or on-sky (the density profile is still matched to the data, and only stars with 60 < Rgal < 100 kpc are included). a, Projection in the usual Galactic coordinates. b, Projection in Galactocentric coordinates (what an observer would see if placed at the Galactic Centre). In b, the model amplitude of the collective response is asymmetric and is largest near the Galactic plane.
Extended Data Fig. 9 Predicted density distribution of a tilted stellar halo.
All-sky density distribution of a smooth triaxial model stellar halo that is tilted by 60° along the y axis. While this model captures some of the qualitative behaviour seen in the data (Fig. 1a), it fails to reproduce both the detailed shape of the local wake and predicts a precise symmetry in the north and south, which is not observed.
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Conroy, C., Naidu, R.P., Garavito-Camargo, N. et al. All-sky dynamical response of the Galactic halo to the Large Magellanic Cloud. Nature 592, 534–536 (2021). https://doi.org/10.1038/s41586-021-03385-7
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DOI: https://doi.org/10.1038/s41586-021-03385-7
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