The Proper Motion of Draco II with HST using Multiple Reference Frames and Methodologies

TL;DR

This paper measures the proper motion of Draco II, an ultra-faint dwarf galaxy, using HST data and multiple reference frames, achieving the most precise measurement by incorporating background galaxies.

Contribution

It introduces a methodology combining Gaia stars and background galaxies as reference frames, improving proper motion accuracy for low-luminosity systems.

Findings

Proper motion of Draco II measured as (1.043±0.029, 0.879±0.028) mas/yr.

Using background galaxies enhances measurement precision by up to 2×.

All three reference frame methods yield consistent proper motion results.

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 $(\mu_{\alpha}^*, \mu_{\delta}) = (1.043\pm0.029, 0.879\pm0.028)$ mas/yr, which is the most precise measurement of the three we present in this paper.