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. 
Marius Cautun and Carlos S. Frenk (ICC Durham)
astro-ph.GA astro-ph.CO