Gaia EDR3 proper motions of Milky Way dwarfs I: 3D Motions and Orbits

TL;DR

Using Gaia EDR3 data, the study estimates precise 3D motions and orbits of 46 Milky Way dwarf galaxies, revealing their orbital distribution and potential implications for galaxy formation and structure.

Contribution

This paper provides improved proper motion measurements and orbit calculations for Milky Way dwarf spheroidals, highlighting their orbital clustering and potential association with the Vast Polar Structure.

Findings

Proper motions are 2.5 times more precise than Gaia DR2.

Pericenter distances are well constrained regardless of Milky Way mass model.

Most dSphs are concentrated near their pericenters and align with the VPOS.

Abstract

Based on Gaia Early Data Release 3 (EDR3), we estimate the proper motions for 46 dwarf spheroidal galaxies (dSphs) of the Milky Way. The uncertainties in proper motions, determined by combining both statistical and systematic errors, are smaller by a factor 2.5, when compared with Gaia Data Release 2. We have derived orbits in four Milky Way potential models that are consistent with the MW rotation curve, with total mass ranging from $2.8\times10^{11}$ $M_{\odot}$ to $15\times10^{11}$ $M_{\odot}$. Although the type of orbit (ellipse or hyperbola) are very dependent on the potential model, the pericenter values are firmly determined, largely independent of the adopted MW mass model. By analyzing the orbital phases, we found that the dSphs are highly concentrated close to their pericenter, rather than to their apocenter as expected from Kepler's law. This may challenge the fact that most dSphs are Milky Way satellites, or alternatively indicates an unexpected large number of undiscovered dSphs lying very close to their apocenters. Between half and two thirds of the satellites have orbital poles that indicate them to orbit along the Vast Polar Structure (VPOS), with the vast majority of these co-orbiting in a common direction also shared by the Magellanic Clouds, which is indicative of a real structure of dSphs.