The impact of Gaia on our understanding of the Vast Polar Structure of the Milky Way

Abstract: The Milky Way (MW) is surrounded by a Vast Polar Structure (VPOS) of satellite galaxies, star clusters, and streams. Proper motion measurements for the brightest MW satellites indicate that they are predominantly co-orbiting along the VPOS. This is consistent with a dynamically stable structure. Assuming that all satellites that are aligned with the VPOS also co-orbit along this structure allows to empirically predict their systemic proper motions. Testing predictions for individual satellite galaxies at large distances requires high-accuracy proper motion measurements such as with the Hubble Space Telescope. However, for nearby MW satellites, Gaia will allow to test these predictions, in particular for a statistical sample of satellites since proper motion predictions exist for almost all of them. This will clarify how rotationally supported the VPOS is. In addition, Gaia will discover Galactic substructure, in particular stellar streams. The degree of their alignment with the VPOS might further constrain its stability and nature.