The Link between Galactic Satellite Orbits and Subhalo Accretion

We calculate the orbital angular momentum of dark matter subhaloes in the Aquarius simulations of cold dark matter galactic haloes. We calculate the orientation of their angular momentum relative to that of the spin vector of their host halo and find a variety of different configurations. All six Aquarius haloes contain statistically significant populations of subhalo orbits that are aligned with the main halo spin. All haloes posses a population of subhaloes that rotates in the same direction as the main halo and three of them possess, in addition, a population that rotates in the opposite direction. These configurations arise from the filamentary accretion of subhaloes. Quasi-planar distributions of coherently rotating satellites, such as those inferred in the Milky Way and other galaxies, arise naturally in simulations of a $\Lambda$CDM universe.