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The Milky Way (MW) is surrounded by numerous satellite objects: dwarf galaxies, globular clusters and streams of disrupted systems. Together, these form a vast polar structure (VPOS), a thin plane spreading to Galactocentric distances as large as 250 kpc. The orbital directions of satellite galaxies and the preferred alignment of streams with the VPOS demonstrate that the objects orbit within the structure. This strong phase-space correlation is at odds with the expectations from simulations of structure formation based on the cold dark matter cosmology (LCDM). The accretion of sub-halos along filaments has been suggested as the origin of the anisotropic distribution. We have tested this scenario using the results of high-resolution cosmological simulations and found it unable to account for the large degree of correlation of the MW satellite orbits. It is therefore advisable to search for alternative explanations. The formation of tidal dwarf galaxies (TDGs) in the debris expelled from interacting galaxies is a very natural formation scenario of the VPOS. If a number of MW satellites truly are TDGs, mistakenly interpreting them to trace the dark-matter sub-structure of the MW halo would significantly enhance the 'small-scale' problems which are already known to plague the LCDM model.

THE VAST POLAR STRUCTURE OF THE MILKY WAY AND FILAMENTARY ACCRETION OF SUB-HALOS