2015MNRAS.452L..41E

Walker et al.'s Magellan/Michigan/MIKE Fiber System (MMFS) survey identified 1355 red giant candidates in the dwarf spheroidal galaxy Sculptor. We find that the Gaia satellite will be able to measure the proper motions of 139 of these with a precision of between 13 and 20 km/s. Using a Jeans analysis and 5-parameter density model we show that this allows a determination of the mass within the deprojected half-light radius to within 16 percent and a measurement of the dark matter density exponent γ to within 0.68 within that radius. If, even at first light, the Thirty Meter Telescope (TMT) observes Sculptor then the combined observations will improve the precision on these proper motions to about 5 km/s, about 5 years earlier than would be possible without Gaia, further improving the precision of γ to 0.27. Using a bimodal stellar population model for Sculptor the precision of γ improves by about 30 percent. This suggests that Gaia (with TMT) is capable of excluding a cusped profile of the kind predicted by CDM simulations with 2σ (4σ) of confidence.