GEPI

The Milky Way satellite galaxies are part of a vast polar structure
(VPOS), a thin plane also consisting of globular clusters and streams
of disrupted systems. I will present an updated analysis of the
orbital poles of the satellites based on their proper motions,
revealing that most satellite galaxies co-orbit within the VPOS. In
addition, it was recently discovered that about half of the satellite
galaxies of the Andromeda galaxy define and mostly co-orbit in a thin
plane. I will illustrate the relative orientations of the dwarf galaxy
planes with a 3D-model, revealing a surprising amount of spacial order
in the Local Group. The existence of similar, co-rotating phase-space
structures in the two only satellite galaxy systems for which 3D
positions are known emphasizes the need to develop an understanding of
their origin. I will discuss why suggested formation scenarios which
interpret the satellite galaxies as tracers of dark matter sub-halos
being accreted along cosmic filaments fail to deliver a satisfactory
explanation. As an alternative origin I suggest the formation of tidal
dwarf galaxies (TDGs) in the phase-space correlated debris of
interacting galaxies. However, this has far-reaching consequences. If
a number of local dwarf galaxies are ancient TDGs, spuriously
interpreting them to trace the dark matter sub-structure gives rise to
inconsistencies with cosmological predictions, artificially attenuates
the missing satellites problem and therefore seriously affects current
near-field cosmology.