GEPI

It has been known for a long time that the satellite galaxies of the Milky Way (MW) show a significant amount of phase-space correlation : they are distributed in a highly inclined Disc of Satellites (DoS). We extend the previous studies on the DoS by analyzing for the first time the orientations of streams of stars and gas, and the distributions of globular clusters within the halo of the MW. It is shown that the spatial distribution of MW globular clusters classified as young halo clusters is similar to the DoS and that half of the analyzed streams align well with the DoS. The MW is surrounded by a vast polar structure (VPOS) of subsystems (satellite galaxies, globular clusters and streams), spreading from Galactocentric distances as small as 10 kpc out to 250 kpc.

The formation of the observed VPOS is extremely improbable when assuming an isotropic infall of cosmological sub-structure components onto the MW. Either a large number of infalling objects had to be highly correlated, or their majority had to be formed as a single population, as is possible only in tidal material. Tidal debris in numerical models of galaxy interactions reproduce the observed phase-space properties of the VPOS. The potential consequences of the MW satellites being tidal dwarf galaxies are severe. If all satellite galaxies and young halo globular clusters in the VPOS are of tidal origin, then the MW does not have any luminous dark-matter substructures and the missing satellites problem becomes a catastrophic failure.