The origin and implications of dark matter anisotropic cosmic infall on ≈ L* haloes

We measure the anisotropy of dark matter flows on small scales (similar to500 kpc) in the near environment of haloes using a large set of simulations. We rely on two different approaches to quantify the anisotropy of the cosmic infall: we measure the flows at the virial radius of the haloes while describing the infalling matter via fluxes through a spherical shell; and we measure the spatial and kinematical distributions of satellites and substructures around haloes detected by the subclump finder ADAPTAHOP described for the first time in the appendix. The two methods are found to be in agreement both qualitatively and quantitatively via one- and two-point statistics. The peripheral and advected momenta are correlated with the spin of the embedded halo at levels of 30 and 50 per cent. The infall takes place preferentially in the plane perpendicular to the direction defined by the spin of the halo. We computed the excess of equatorial accretion both through rings and via a harmonic expansion of the infall. The level of anisotropy of infalling matter is found to be 15 per cent. The substructures have their spin orthogonal to their velocity vector in the rest frame of the halo at a level of about 5 per cent, suggestive of an image of a flow along filamentary structures, which provides an explanation for the measured anisotropy. Using a 'synthetic' stacked halo, it is shown that the positions and orientations of satellites relative to the direction of spin of the halo are not random even in projection. The average ellipticity of stacked haloes is 10 per cent, while the alignment excess in projection reaches 2 per cent. All measured correlations are fitted by a simple three-parameter model. We conclude that a halo does not see its environment as an isotropic perturbation, we investigate how the anisotropy is propagated inwards using perturbation theory, and we discuss briefly the implications for weak lensing, warps and the thickness of galactic discs.