The tangential velocity excess of the Milky Way satellites
  We estimate the systemic orbital kinematics of the Milky Way classical
satellites and compare them with predictions from the \Lambda{} cold dark
matter (\Lambda{}CDM) model derived from a semi-analytical galaxy formation
model applied to high resolution cosmological N-body simulations. We find that
the Galactic satellite system is atypical of \Lambda{}CDM systems. The subset
of 10 Galactic satellites with proper motion measurements has a velocity
anisotropy, \beta=-2.2$\pm$0.4, that lies in the 2.9% tail of the \Lambda{}CDM
distribution. Individually, the Milky Way satellites have radial velocities
that are lower than expected for their proper motions, with 9 out of the 10
having at most 20% of their orbital kinetic energy invested in radial motion.
Such extreme values are expected in only 1.5% of \Lambda{}CDM satellites
systems. This tangential motion excess is unrelated to the existence of a
Galactic "disc of satellites". We present theoretical predictions for larger
satellite samples that may become available as more proper motion measurements
are obtained.
