Event

Comparisons of observed satellite galaxies with CDM
simulations have revealed numerous 'small-scale' problems. While the
missing-satellites, the core-cusp and potentially even the
too-big-to-fail problems might be solved by invoking baryonic
processes such as feedback not incorporated in the initial
simulations, the problem I will present lacks a viable solution in
the CDM framework and became increasingly severe in recent years.
The satellites of the Milky Way and of the Andromeda galaxy
preferentially co-orbit within narrow planes, the Vast Polar
Structure (VPOS) of the Milky Way and the Great Plane of Andromeda
(GPoA). In addition, there is an increasing number of indications
that such satellite structures are present beyond the Local Group.
Sixty per cent or more of all satellites might be part of co-orbiting
planes. I will show that such highly anisotropic phase-space
distribution of sub-halos are extremely rare in cosmological
simulations. The satellite phase-space correlation might be the
most-serious small-scale problem for CDM because the distribution
of sub-halos is dominated by gravitational effects and not strongly
affected by baryonic physics. Feedback processes act on scales of
individual satellites, while the satellite alignments are present on
scales of many 100 kpc. Scenarios explaining the observed coherence
of satellite positions and orbits exist, but require radical
adjustments to the CDM model.