A portrait of the vast polar structure as a young phenomenon: Hints from its member satellites

Abstract

Context. It has been observed that several Milky Way (MW) satellite dwarf galaxies are distributed along a coherent planar distribution known as the vast polar structure (VPOS). Aims. Here we investigate whether MW satellites located on the VPOS have different physical and orbital properties from those not associated with it. Methods. Using the proper motion measurements of the MW satellites from the Gaia mission and literature values for their observational parameters, we first discriminate between systems that may or may not be associated with the VPOS, and then compare their chemical and dynamical properties. Results. Comparing the luminosity distributions of the on-plane and off-plane samples, we find an excess of bright satellites observed on the VPOS. Despite this luminosity gap, we do not observe a significant preference for on-plane or off-plane systems to follow different scaling relations. The on-plane systems also show a striking pattern in their radial velocities and orbital phases: almost all co-orbiting satellites are approaching their pericentre, while both counter-orbiting ones are leaving their last pericentre. This is in contrast to the more random distribution of the off-plane sample. The on-plane systems also tend to have the lowest orbital energies for a given value of angular momentum. These results are robust to the assumed MW potential, even in the case of a potential perturbed by the arrival of a massive Large Magellanic Cloud. Considering them a significant property of the VPOS, we explore several scenarios, all related to the late accretion of satellite systems, which interpret the VPOS as a young structure. Conclusions. From the results obtained, we hypothesise that the VPOS formed as a result of the accretion of a group of dwarf galaxies. More accurate proper motions and dedicated studies in the context of cosmological simulations are needed to confirm this scenario.