The accretion of dark matter subhaloes within the cosmic web: primordial anisotropic distribution and its universality

The distribution of galaxies displays anisotropy on different scales and it is often referred as galaxy alignment. To understand the origin of galaxy alignments on small scales, one must investigate how galaxies were accreted in the early universe and quantify their primordial anisotropic at the time of accretion. In this paper we use N-body simulations to investigate the accretion of dark matter subhaloes, focusing on their alignment with the host halo shape and the orientation of mass distribution on large scale, defined using the hessian matrix of the density field. The large/small (e1/e3) eigenvalues of the hessian matrix define the fast/slow collapse direction of dark matter on large scale. We find that: 1) the halo major axis is well aligned with the e3 (slow collapse) direction, and it is stronger for massive haloes; 2) subhaloes are predominately accreted along the major axis of the host halo, and the alignment increases with the host halo mass. Most importantly, this alignment is universal; 3) accretion of subhaloes with respect to the e3 direction is not universal. In massive haloes, subhaloes are accreted along the e3 (even stronger than the alignment with the halo major axis), but in low-mass haloes subhaloes are accreted perpendicular to the e3. The transit mass is lower at high redshift. The last result well explains the puzzled correlation (both in recent observations and simulations) that massive galaxies/haloes have their spin perpendicular to the filament, and the spin of low-mass galaxies/haloes is slightly aligned with the filament, under the assumption that the orbital angular momentum of subhaloes is converted to halo spin.