Proper Motion of the Draco Dwarf Galaxy from Subaru Suprime-Cam Data
  We have measured the absolute proper motion of the Draco dwarf spheroidal
galaxy using Subaru Suprime-Cam images taken at three epochs, with time
baselines of 4.4 and 7 years. The magnitude limit of the proper-motion study is
i=25, thus allowing for thousands of background galaxies and Draco stars to be
used to perform extensive astrometric tests and to derive the correction to an
inertial reference frame. The derived proper motion is (mu_alpha, mu_delta)
=(-0.284 +- 0.047, -0.289 +-0.041) mas/yr. This motion implies an orbit that
takes Draco to a pericenter of $\sim 20$ kpc; a somewhat disruptive orbit
suggesting that tides might account for the rising velocity-dispersion profile
of Draco seen in line-of-sight velocity studies. The orbit is only marginally
consistent with Draco's membership to the vast polar structure of Galactic
satellites, in contrast to a recent HST proper-motion measurement that finds
alignment very likely. Our study is a test case to demonstrate that deep
imaging with mosaic cameras of appropriate resolution can be used for
high-accuracy, ground-based proper-motion measurement. As a useful by-product
of the study, we also identify two faint brown-dwarf candidates in the
foreground field.
