Gaia EDR3 Proper Motions of Milky Way Dwarfs. I. 3D Motions and Orbits

Based on Gaia Early Data Release 3 (EDR3), we estimate the proper motions of 46 dwarf galaxies of the Milky Way (MW). The uncertainties in proper motions, determined by combining both statistical and systematic errors, are smaller by a factor of 2.5 when compared with Gaia Data Release 2. We have derived orbits in four MW potential models that are consistent with the MW rotation curve, with total mass ranging from 2.8 × 10¹¹ M_⊙ to 15 × 10¹¹ M_⊙. 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 dwarf galaxies 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 dwarf galaxies are MW satellites, or alternatively indicates an unexpectedly large number of undiscovered dwarf galaxies 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, 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 dwarf galaxies.