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<SUP>1,2</SUP>. Even for the dwarf galaxy Sculptor—one of the best-studied systems, which is inferred to be strongly dark matter dominated<SUP>3,4</SUP>—there are conflicting reports<SUP>5-7</SUP> 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<SUP>8</SUP> 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<SUB>R</SUB> = 11.5 ± 4.3 km s<SUP>-1</SUP> and σ<SUB>T</SUB> = 8.5 ± 3.2 km s<SUP>-1</SUP> 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 <mml:mstyle fontfamily="Whitney Semibold"> 0.8 6<SUB>-0.83</SUB><SUP>+0.12</SUP></mml:mstyle> at a location beyond the core radius. Taken at face value, this high radial anisotropy requires abandoning conventional models<SUP>9</SUP> for Sculptor's mass distribution. Our sample is dominated by metal-rich stars and for these we find <mml:mstyle fontfamily="Whitney semibold">β<SUP>M R</SUP> 0.9 5<SUB>-0.27</SUB><SUP>+0.04</SUP></mml:mstyle>—a value consistent with multi-component spherical models where Sculptor is embedded in a cuspy dark halo<SUP>10</SUP>, as might be expected for cold dark matter.