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

Abstract

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^1,2. Even for the dwarf galaxy Sculptor—one of the best-studied systems, which is inferred to be strongly dark matter dominated^3,4—there are conflicting reports^5,6,7 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.