The power of teaming up HST and Gaia: the first proper motion measurement of the distant cluster NGC 2419

Aims: We present the first measurement of the proper motion and orbit of the very distant and intriguing globular cluster NCG 2419.
Methods: We have combined data from HST and Gaia DR1 to derive the relative proper motions of stars in the direction of the cluster. To tie these to an absolute reference frame, we have used a background galaxy located in the field.
Results: We find the absolute proper motion of NGC 2419 to be (μ_αcos(δ), μ_δ) = (-0.17 ± 0.26,-0.49 ± 0.17) mas yr^-1. We have integrated the orbit of the cluster in a Galactic potential and found it to oscillate between 53 kpc and 98 kpc on an almost polar orbit. This, and the fact that it shares the same sense of rotation around the Milky Way, make it very likely that NGC 2419 is a former cluster of the Sagittarius dwarf spheroidal galaxy.