Proper Motion of the Draco Dwarf Galaxy Based On Hubble Space Telescope Imaging

Understand

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$^{-1}$.