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

Abstract

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