Three-dimensional 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 1,2 . Even for the dwarf galaxy Sculptor which is one of the best studied systems and 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 3D internal motions of its stars have never been measured. Here we report, based on data from the Gaia space mission 8 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 highinclination 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 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, implying that 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 such a high radial anisotropy requires abandoning conventional models 9 for the mass distribution in Sculptor. Our sample is dominated by metal-rich stars and for these we find β M R ∼ 0.95−0.27 , a value consistent with multi-component models where Sculptor is embedded in a cuspy dark halo 10 as expected for cold dark matter.To measure the proper motions (PMs) of individual stars in Sculptor we used data taken 12.27 years apart. The first epoch was acquired with the Advanced Camera for Surveys on board HST. The data set consists of two overlapping pointings separated by about 2 ′ (∼ 50 pc, see Fig. 1), each split in several 400 sec exposures in the F775W filter. The overlapping field-of-view has been observed 11 times. We obtained a catalog of positions, instrumental magnitudes and Point Spread Function (PSF) fitting-quality parameters by treating each chip of each exposure independently. Stellar positions were corrected for filter-dependent geometric distortions 11 . We then cross-matched the single catalogs to compute 3σ-clipped * Corresponding author: massariATastro.rug.nl -2 -average positions, magnitudes and corresponding uncertainties. We built the complete HST catalog after excluding all the saturated sources and those that were measured less than 4 times. The second epoch is provided by the Gaia first data release 12 . We extracted from the Gaia archive all sources in the direction of Sculptor.We transformed the HST positions to the equatorial reference frame defined by the Gaia data (right ascension, RA, and declination, DEC), using a six-parameter linear transformation 14 . We found 126 stars in common and their PMs were computed as the difference between the Gaia and HST positions, divided by the temporal baseline. The uncertainties on the PMs were computed as the sum in quadrature between the Gaia and HST positional e...

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