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Keywords:

  • globular clusters: general;
  • Galaxy: halo;
  • Galaxy: structure;
  • galaxies: interactions;
  • galaxies: kinematics and dynamics;
  • Local Group

ABSTRACT

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, and they are distributed in a highly inclined disc of satellites (DoS). We have extended the previous studies on the DoS by analysing 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 (YH GC) is very similar to that of the DoS, while seven of the 14 analysed streams align with the DoS. The probability to find the observed clustering of streams is only 0.3 per cent when assuming isotropy. The MW thus 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. These findings demonstrate that a near-isotropic infall of cosmological substructure components on to the MW is essentially ruled out because a large number of infalling objects would have had to be highly correlated, to a degree not natural for dark matter substructures. The majority of satellites, streams and YH GCs had to be formed as a correlated population. This is possible in tidal tails consisting of material expelled from interacting galaxies. We discuss the tidal scenario for the formation of the VPOS, including successes and possible challenges. The potential consequences of the MW satellites being tidal dwarf galaxies are severe. If all the satellite galaxies and YH GCs have been formed in an encounter between the young MW and another gas-rich galaxy about 10–11 Gyr ago, then the MW does not have any luminous dark matter substructures and the missing satellites problem becomes a catastrophic failure of the standard cosmological model.