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 to the cluster. To tie 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 $(\mu_{\alpha}\cos(\delta)$,
$\mu_{\delta}$)=($-0.17\pm0.26,-0.49\pm0.17$) mas/yr. We have integrated the
orbit of the cluster in a Galactic potential and found it to oscillate between
$\sim$53 kpc and $\sim$98 kpc on a nearly polar orbit. This makes it very
likely that NGC 2419 is a former cluster of the Sagittarius dwarf spheroidal
galaxy, also because it shares the same sense of rotation around the Milky Way.
