The rotationally stabilized VPOS and predicted proper motions of the Milky Way satellite galaxies

The satellite galaxies of the Milky Way (MW) define a vast polar structure (VPOS), a thin plane perpendicular to the MW disc. Proper motion (PM) measurements are now available for all of the 11 brightest, `classical' satellites and allow an updated analysis of the alignment of their orbital poles with this spatial structure. The coherent orbital alignment of 7 to 9 out of 11 satellites demonstrates that the VPOS is a rotationally stabilized structure and not only a pressure-supported, flattened ellipsoid. This allows us to empirically and model independently predict the PMs of almost all satellite galaxies by assuming that the MW satellite galaxies orbit within the VPOS. As a test of our method, the predictions are best met by satellites whose PMs are already well constrained, as expected because more uncertain measurements tend to deviate more from the true values. Improved and new PM measurements will further test these predictions. A strong alignment of the satellite galaxy orbital poles is not expected in dark matter based simulations of galaxy formation. Coherent orbital directions of satellite galaxies are, however, a natural consequence of tidal dwarf galaxies formed together in the debris of a galaxy collision. The orbital poles of the MW satellite galaxies therefore lend further support to tidal scenarios for the origin of the VPOS and are a very significant challenge for the standard LCDM model of cosmology. We also note that the dependence of the MW satellite speeds on Galactocentric distance appear to map an effective potential with a constant velocity of approximately 240 km/s to about 250 kpc. The individual satellite velocities are only mildly radial.Comment: 17 pages, 8 figures, 4 tables, accepted for publication in MNRA

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

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 degrees and the VPOS flattening is almost unchanged (31 kpc height). Interestingly, the shortest distance of the VPOS plane from the MW center is now only 2.5 kpc, indicating that the new discoveries balance out the VPOS at the Galactic center. The vast majority of the MW satellites 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 years 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 halos, and (iii) obey MOND.Comment: 17 pages, 6 figures, 5 tables. Updated to match version accepted for publication in MNRA