The Intrinsic Alignment of Dark Halo Substructures

We investigate the intrinsic alignments of dark halo substructures with theirhost halo major-axis orientations both analytically and numerically.Analytically, we derive the probability density distribution of the anglesbetween the minor axes of the substructures and the major axes of their hosthalos from the physical principles, under the assumption that the substructurealignment on galaxy scale is a consequence of the tidal fields of the host halogravitational potential. Numerically, we use a sample of four cluster-scalehalos and their galaxy-scale substructures from recent high-resolution N-bodysimulations to measure the probability density distribution. We compare thenumerical distribution with the analytic prediction, and find that the tworesults agree with each other very well. We conclude that our analytic modelprovides a quantitative physical explanation for the intrinsic alignment ofdark halo substructures. We also discuss the possibility of discriminating ourmodel from the anisotropic infall scenario by testing it against very largeN-body simulations in the future.Comment: accepted version, ApJL in press, minor revision, 12 pages, 2 figure