Revised and new proper motions for confirmed and candidate Milky Way dwarf galaxies

TL;DR

This paper presents a new method to derive systemic proper motions for Milky Way dwarf galaxy satellites using Gaia DR2 data, providing first-time measurements for several objects and improving accuracy over previous studies.

Contribution

It introduces a novel maximum likelihood approach that simultaneously analyzes spatial, color-magnitude, and proper motion data without requiring confirmed radial velocity members.

Findings

Derived proper motions for 59 satellites, including first measurements for five.

Achieved uncertainties about 1.4 times smaller than previous literature.

Successfully identified member stars with less than 5% contamination.

Abstract

A new derivation of systemic proper motions of Milky Way satellites is presented, and applied to 59 confirmed or candidate dwarf galaxy satellites using Gaia Data Release 2. This constitutes all known Milky Way dwarf galaxies (and likely candidates) as of May 2020 except the Magellanic Clouds, the Canis Major and Hydra 1 stellar overdensities, and the tidally disrupting Bootes III and Sagittarius dwarf galaxies. We derive systemic proper motions for the first time for Indus 1, DES J0225+0304, Cetus 2, Pictor 2 and Leo T, but note that the latter three rely on photometry that is of poorer quality than for the rest of the sample. We cannot resolve a signal for Bootes 4, Cetus 3, Indus 2, Pegasus 3, or Virgo 1. Our method is inspired by the maximum likelihood approach of Pace & Li (2019) and examines simultaneously the spatial, color-magnitude, and proper motion distribution of sources. Systemic proper motions are derived without the need to identify confirmed radial velocity members, although the proper motions of these stars, where available, are incorporated into the analysis through a prior on the model. The associated uncertainties on the systemic proper motions are on average a factor of $\sim 1.4$ smaller than existing literature values. Analysis of the implied membership distribution of the satellites suggests we accurately identify member stars with a contamination rate less than 1 in 20.