The VPOS: a vast polar structure of satellite galaxies, globular clusters and streams around the Milky Way

Abstract

It has been known for a long time that the satellite galaxies of the Milky\nWay (MW) show a significant amount of phase-space correlation, they are\ndistributed in a highly inclined Disc of Satellites (DoS). We have extended the\nprevious studies on the DoS by analysing for the first time the orientations of\nstreams of stars and gas, and the distributions of globular clusters within the\nhalo of the MW. It is shown that the spatial distribution of MW globular\nclusters classified as young halo clusters (YH GC) is very similar to the DoS,\nwhile 7 of the 14 analysed streams align with the DoS. The probability to find\nthe observed clustering of streams is only 0.3 per cent when assuming isotropy.\nThe MW thus is surrounded by a vast polar structure (VPOS) of subsystems\n(satellite galaxies, globular clusters and streams), spreading from\nGalactocentric distances as small as 10 kpc out to 250 kpc. These findings\ndemonstrate that a near-isotropic infall of cosmological sub-structure\ncomponents onto the MW is essentially ruled out because a large number of\ninfalling objects would have had to be highly correlated, to a degree not\nnatural for dark matter sub-structures. The majority of satellites, streams and\nYH GCs had to be formed as a correlated population. This is possible in tidal\ntails consisting of material expelled from interacting galaxies. We discuss the\ntidal scenario for the formation of the VPOS, including successes and possible\nchallenges. The potential consequences of the MW satellites being tidal dwarf\ngalaxies are severe. If all the satellite galaxies and YH GCs have been formed\nin an encounter between the young MW and another gas-rich galaxy about 10-11\nGyr ago, then the MW does not have any luminous dark-matter substructures and\nthe missing satellites problem becomes a catastrophic failure of the standard\ncosmological model.\n