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

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

Publication date: 27 November 2017

Authors: Massari, D., et al.

Journal: Nature Astronomy
Year: 2017

Copyright: © 2017 Macmillan Publishers Limited, part of Springer Nature.

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 matter. Even for the dwarf galaxy Sculptor–one of the best-studied systems, which is inferred to be strongly dark matter dominated–there are conflicting reports 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 mission 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 σR = 11.5 ± 4.3 km s−1 and σT = 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 β~0.86+0.12-0.83 at a location beyond the core radius. Taken at face value, this high radial anisotropy requires abandoning conventional models for Sculptor's mass distribution. Our sample is dominated by metal-rich stars and for these we find βMR~0.95+0.04-0.27–a value consistent with multi-component spherical models where Sculptor is embedded in a cuspy dark halo, as might be expected for cold dark matter.

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