Proper Motion of the Draco Dwarf Galaxy from Subaru Suprime-Cam Data

Abstract

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.