The kinematics of globular clusters systems in the outer halos of the
  Aquarius simulations
  Stellar halos and globular cluster (GC) systems contain valuable information
regarding the assembly history of their host galaxies. Motivated by the
detection of a significant rotation signal in the outer halo GC system of M31,
we investigate the likelihood of detecting such a rotation signal in
projection, using cosmological simulations. To this end we select subsets of
tagged particles in the halos of the Aquarius simulations to represent mock GC
systems, and analyse their kinematics. We find that GC systems can exhibit a
non-negligible rotation signal provided the associated stellar halo also has a
net angular momentum. The ability to detect this rotation signal is highly
dependent on the viewing perspective, and the probability of seeing a signal
larger than that measured in M31 ranges from 10% to 90% for the different halos
in the Aquarius suite. High values are found from a perspective such that the
projected angular momentum of the GC system is within 40 deg of the rotation
axis determined via the projected positions and line-of-sight velocities of the
GCs. Furthermore, the true 3D angular momentum of the outer stellar halo is
relatively well aligned, within 35 deg, with that of the mock GC systems. We
argue that the net angular momentum in the mock GC systems arises naturally
when the majority of the material is accreted from a preferred direction,
namely along the dominant dark matter filament of the large-scale structure
that the halos are embedded in. This, together with the favourable edge-on view
of M31's disk suggests that it is not a coincidence that a large rotation
signal has been measured for its outer halo GC system.
