3D motions in the Sculptor dwarf galaxy as a glimpse of a new era

Abstract

The 3D motions of stars in small galaxies beyond our own are minute and yet they are crucial for our understanding of the nature of gravity and dark matter. Even for the dwarf galaxy Sculptor which is one of the best studied systems and inferred to be strongly dark matter dominated, there are conflicting reports on its mean motion around the Milky Way and the 3D internal motions of its stars have never been measured. Here we report, based on data from the Gaia space mission and the Hubble Space Telescope, a new precise measurement of Sculptor's mean proper motion. From this we deduce that Sculptor is currently at its closest approach to the Milky Way and moving on an elongated high-inclination orbit that takes it much farther away than previously thought. For the first time we are also able to measure the internal motions of stars in Sculptor. We find ${\sigma }_R=11.5\pm 4.3$ km/s and ${\sigma }_T=8.5\pm 3.2$ km/s along the projected radial and tangential directions, implying that the stars in our sample move preferentially on radial orbits as quantified by the anisotropy parameter, which we find to be $\beta \sim 0.86_{-0.83}^{+0.12}$ at a location beyond the core radius. Taken at face value such a high radial anisotropy requires abandoning conventional models for the mass distribution in Sculptor. Our sample is dominated by metal-rich stars and for these we find ${\beta }^{MR}\sim 0.95_{-0.27}^{+0.04}$, a value consistent with multi-component models where Sculptor is embedded in a cuspy dark halo as expected for cold dark matter.

Accepted for publication in Nature Astronomy. Note press embargo until 16:00 London time / 11:00 US Eastern Time on 27 November 2017

References in corpus (14)

• The Gaia mission
• Gaia Data Release 1. Summary of the astrometric, photometric, and survey properties
• Gaia Data Release 1: Astrometry - one billion positions, two million proper motions and parallaxes
• Gaia Data Release 1: Catalogue validation
• The kinematic status and mass content of the Sculptor dwarf spheroidal galaxy
• Velocity Dispersion Profiles of Seven Dwarf Spheroidal Galaxies
• Stellar Velocities in the Carina, Fornax, Sculptor and Sextans dSph Galaxies: Data from the Magellan/MMFS Survey
• Constraining the Galaxy's dark halo with RAVE stars
• Hubble Space Telescope Proper Motion (HSTPROMO) Catalogs of Galactic Globular Clusters. I. Sample Selection, Data Reduction and NGC 7078 Results
• The M31 Velocity Vector. I. Hubble Space Telescope Proper Motion Measurements
• Dwarf Spheroidals in MOND
• Systemic Proper Motions of Milky Way Satellites from Stellar Redshifts: the Carina, Fornax, Sculptor and Sextans Dwarf Spheroidals
• Determining the Nature of Dark Matter with Astrometry
• The power of teaming up HST and Gaia: the first proper motion measurement of the distant cluster NGC 2419

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• Scale-Invariant Dynamics of Galaxies, MOND, Dark Matter, and the Dwarf Spheroidals
• Ultracompact minihalos associated with stellar-mass primordial black holes
• Distribution of Phantom Dark Matter in Dwarf Spheroidals