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

TL;DR

This study measures the proper motion of the Draco dwarf galaxy using Hubble Space Telescope imaging over two years, providing insights into its velocity and orbital inclination relative to the Milky Way.

Contribution

First precise proper motion measurement of Draco dwarf galaxy using HST data, enabling detailed orbital analysis in the Galactic context.

Findings

Proper motion in equatorial and Galactic coordinates determined.

Galactic rest-frame proper motion calculated after Sun and LSR correction.

Orbital inclination and velocity suggest alignment with the VPOS structure.

Abstract

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.