Observational evidence for the radial alignment of satellites with their darkmatter host has been accumulating steadily in the past few years. The effect isseen over a wide range of scales, from massive clusters of galaxies down togalaxy-sized systems, yet the underlying physical mechanism has still not beenestablished. To this end, we have carried out a detailed analysis of the shapesand orientations of dark matter substructures in high-resolution N-bodycosmological simulations. We find a strong tendency for radial alignment of thesubstructure with its host halo: the distribution of halo major axes is veryanisotropic, with the majority pointing towards the center of mass of the host.The alignment peaks once the sub-halo has passed the virial radius of the hostfor the first time, but is not subsequently diluted, even after the halos havegone through as many as four pericentric passages. This evidence points to theexistence of a very rapid dynamical mechanism acting on these systems and weargue that tidal torquing throughout their orbits is the most likely candidate.
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