The Proper Motion of Draco II with HST using Multiple Reference Frames and Methodologies
Authors
Jack T. Warfield
Kevin A. McKinnon
Sangmo Tony Sohn
Nitya Kallivayalil
Alessandro Savino
Roeland P. van der Marel
Andrew B. Pace
Christopher T. Garling
Niusha Ahvazi
Paul Bennet
Roger E. Cohen
Matteo Correnti
Mark A. Fardal
Kristen B. W. McQuinn
Max J. B. Newman
Eduardo Vitral
Abstract
We present proper motion (PM) measurements for Draco II, an ultra-faint dwarf satellite of the Milky Way. These PMs are measured using two epochs of Hubble Space Telescope Advanced Camera for Surveys (HST/ACS) imaging separated by a 7 year time baseline. Measuring PMs of low-luminosity systems is difficult due to the low number of member stars, requiring a precise inertial reference frame. We construct reference frames using three different sets of external sources: 1) stars with Gaia DR3 data, 2) stationary background galaxies, and 3) a combination of the two. We show that all three reference frames give consistent PM results. We find that for this sparse, low-luminosity regime including background galaxies into the reference frame improves our measurement by up to $\sim2\times$ versus using only Gaia astrometric data. Using 301 background galaxies as a reference frame, we find that Draco II's systemic PM is $(μ_α^*, μ_δ) = (1.043\pm0.029, 0.879\pm0.028)$ mas/yr, which is the most precise measurement of the three we present in this paper.
