The new Milky Way satellites: Alignment with the VPOS and predictions for proper motions and velocity dispersions

Abstract

The evidence that stellar systems surrounding the Milky Way (MW) are distributed in a Vast Polar Structure (VPOS) may be observationally biased by satellites detected in surveys of the northern sky. The recent discoveries of more than a dozen new systems in the Southern hemisphere thus constitute a critical test of the VPOS phenomenon. We report that the new objects are located close to the original VPOS, with half of the sample having offsets less than 20 kpc. The positions of the new satellite galaxy candidates are so well aligned that the orientation of the revised best-fitting VPOS structure is preserved to within 9°and the VPOS flattening is almost unchanged (31 kpc height). Interestingly, the shortest distance of the VPOS plane from theMWcentre is now only 2.5 kpc, indicating that the new discoveries balance out theVPOS at the Galactic centre. The vast majority of theMWsatellites are thus consistent with sharing a similar orbital plane as the Magellanic Clouds, confirming a hypothesis proposed by Kunkel & Demers and Lynden-Bell almost 40 yr ago. We predict the absolute proper motions of the new objects assuming they orbit within the VPOS. Independent of the VPOS results, we also predict the velocity dispersions of the new systems under three distinct assumptions: that they (i) are dark matter free star clusters obeying Newtonian dynamics, (ii) are dwarf satellites lying on empirical scaling relations of galaxies in dark matter haloes and (iii) obey modified Newtonian dynamics.