Proper Motion of the Draco Dwarf Galaxy Based On Hubble Space Telescope
  Imaging
  We have measured the proper motion of the Draco dwarf galaxy using images at
two epochs with a time baseline of about two years taken with the Hubble Space
Telescope and the Advanced Camera for Surveys. Wide Field Channel 1 and 2
provide two adjacent fields, each containing a known QSO. The zero point for
the proper motion is determined using both background galaxies and the QSOs and
the two methods produce consistent measurements within each field. Averaging
the results from the two fields gives a proper motion in the equatorial
coordinate system of $(\mu_{\alpha},\mu_{\delta}) = (17.7\pm 6.3, -22.1 \pm
6.3)$ mas century$^{-1}$ and in the Galactic coordinate system of
$(\mu_{\ell},\mu_{b}) = (-23.1\pm 6.3, -16.3 \pm 6.3)$ mas century$^{-1}$.
Removing the contributions of the motion of the Sun and of the LSR to the
measured proper motion yields a Galactic rest-frame proper motion of
$(\mu_{\alpha}^{Grf},\mu_{\delta}^{Grf}) = (51.4\pm 6.3, -18.7 \pm 6.3)$ mas
century$^{-1}$ and $(\mu_{\ell}^{Grf},\mu_{b}^{Grf}) = (-21.8\pm 6.3, -50.1 \pm
6.3)$ mas century$^{-1}$. The implied space velocity with respect to the
Galactic center is $(\Pi, \Theta, Z) = (27\pm 14, 89\pm 25, -212\pm 20)$ km
s$^{-1}$. This velocity implies that the orbital inclination is 70 degrees,
with a 95% confidence interval of (59 deg, 80 deg), and that the plane of the
orbit is consistent with that of the vast polar structure (VPOS) of Galactic
satellite galaxies.
