Gaia Data Release 2: The kinematics of globular clusters and dwarf galaxies around the Milky Way

Abstract

Aims. The goal of this paper is to demonstrate the outstanding quality of the 2nd Data Release of the Gaia mission and its power to constrain many different aspects of the dynamics of the satellites of the Milky Way. We focus here on determining the proper motions of 75 Galactic globular clusters, 9 dwarf spheroidal galaxies and one ultra-faint system, and the Large and Small Magellanic Clouds. Methods. Using data extracted from the Gaia archive, we derive the proper motions and parallaxes for these systems, as well as their uncertainties. We demonstrate that the errors, statistical and systematic, are relatively well understood. We have integrated the orbits of these objects in three different Galactic potentials, and characterised their properties. We present the derived proper motions, space velocities and characteristic orbital parameters in various tables to facilitate their use by the astronomical community. Results. Our limited and straightforward analyses have allowed us for example, to: i) determine absolute and very precise proper motions for globular clusters; ii) detect clear rotation signatures in the proper motions of at least 5 globular clusters; iii) show that the satellites of the Milky Way are all on high inclination orbits, but that they do not share a single plane of motion; iv) derive a lower limit for the mass of the Milky Way based on the assumption that the Leo I dwarf spheroidal is bound; v) derive a rotation curve for the LMC based solely on proper motions that is competitive with line-of-sight velocity curves, now using many orders of magnitude more sources; and vi) unveil the dynamical effect of the bar on the motions of stars in the LMC. Conclusions. All these results highlight the incredible power of the Gaia astrometric mission, and in particular of its second Data Release