The cosmic web and the orientation of angular momenta

TL;DR

This study uses a cosmological simulation to analyze how dark matter haloes and their substructures align with the cosmic web, revealing that subhalo orbital angular momentum aligns with large-scale velocity shear and halo spins align with cosmic filaments and sheets.

Contribution

It demonstrates the influence of large-scale velocity shear on the angular momentum orientation of haloes and subhaloes, a novel insight into cosmic web dynamics.

Findings

Subhalo orbital angular momentum aligns with the intermediate eigenvector of the shear tensor.

Halo spin axes tend to align with the third eigenvector in filaments and sheets.

The large-scale velocity shear influences the angular momentum orientation at z=0.

Abstract

We use a 64$h^{-1}$Mpc dark matter (DM) only cosmological simulation to examine the large scale orientation of haloes and substructures with respect the cosmic web. A web classification scheme based on the velocity shear tensor is used to assign to each halo in the simulation a web type: knot, filament, sheet or void. Using $\sim10^6$ haloes that span ~3 orders of magnitude in mass the orientation of the halo's spin and the orbital angular momentum of subhaloes with respect to the eigenvectors of the shear tensor is examined. We find that the orbital angular momentum of subhaloes tends to align with the intermediate eigenvector of the velocity shear tensor for all haloes in knots, filaments and sheets. This result indicates that the kinematics of substructures located deep within the virialized regions of a halo is determined by its infall which in turn is determined by the large scale velocity shear, a surprising result given the virilaized nature of haloes. The non-random nature of subhalo accretion is thus imprinted on the angular momentum measured at z = 0. We also find that haloes' spin axis is aligned with the third eigenvector of the velocity shear tensor in filaments and sheets: the halo spin axis points along filaments and lies in the plane of cosmic sheets.