The VPOS: a vast polar structure of satellite galaxies, globular
  clusters and streams around the Milky Way
  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 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 the DoS,
while 7 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 sub-structure
components onto 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 sub-structures. 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.
