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Three-dimensional motions in the Sculptor dwarf galaxy as a glimpse of a new era


The three-dimensional motions of stars in small galaxies beyond our own are minute, yet they are crucial for understanding the nature of gravity and dark matter1,2. Even for the dwarf galaxy Sculptor—one of the best-studied systems, which is inferred to be strongly dark matter dominated3,4—there are conflicting reports5,6,7 on its mean motion around the Milky Way, and the three-dimensional internal motions of its stars have never been measured. Here, we present precise proper motions of Sculptor’s stars based on data from the Gaia mission8 and Hubble Space Telescope. Our measurements show that Sculptor moves around the Milky Way on a high-inclination elongated orbit that takes it much further out than previously thought. For Sculptor’s internal velocity dispersions, we find σ R = 11.5 ± 4.3 km s−1 and σ T = 8.5 ± 3.2 km s−1 along the projected radial and tangential directions. Thus, the stars in our sample move preferentially on radial orbits as quantified by the anisotropy parameter, which we find to be \({\boldsymbol{\beta }} \sim 0.8{6}_{-0.83}^{+0.12}\) at a location beyond the core radius. Taken at face value, this high radial anisotropy requires abandoning conventional models9 for Sculptor’s mass distribution. Our sample is dominated by metal-rich stars and for these we find \({{\boldsymbol{\beta }}}^{{\rm{M}}R} \sim 0.9{5}_{-0.27}^{+0.04}\)—a value consistent with multi-component spherical models where Sculptor is embedded in a cuspy dark halo10, as might be expected for cold dark matter.

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Fig. 1: Field of view towards the Sculptor dwarf spheroidal galaxy.
Fig. 2: Properties of our sample.
Fig. 3: Comparison with previously published PM estimates for Sculptor.
Fig. 4: Two-dimensional velocity dispersion and orbital anisotropy of Sculptor.

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We made use of data from the European Space Agency mission Gaia (, processed by the Gaia Data Processing and Analysis Consortium ( Funding for the Data Processing and Analysis Consortium was provided by national institutions—in particular, the institutions participating in the Gaia Multilateral Agreement. This work was also based on observations made with the National Aeronautics and Space Administration/European Space Agency HST, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy under National Aeronautics and Space Administration contract NAS 5-26555. A.H. and L.P. acknowledge financial support from a Vici grant from the Netherlands Organisation for Scientific Research. M.A.B. and A.H. are grateful to Nederlandse Onderzoekschool Voor Astronomie for financial support.

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D.M. performed the data analysis and the PM measurements. M.A.B. developed the statistical tools. A.H. derived the relations between observables and orbital anisotropy, coordinated the work and led the scientific interpretation. L.P. performed the orbit computation. A.G.A.B. and E.T. contributed to the presentation of the paper. All authors critically contributed to the work presented here.

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Correspondence to D. Massari or A. Helmi.

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Massari, D., Breddels, M.A., Helmi, A. et al. Three-dimensional motions in the Sculptor dwarf galaxy as a glimpse of a new era. Nat Astron 2, 156–161 (2018).

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