Internal and External Alignment of the Shapes and Angular Momenta of ΛCDM Halos

We investigate how the shapes and angular momenta of galaxy and group mass dark matter halos in a ΛCDM N-body simulation are correlated internally and how they are aligned with respect to the location and properties of surrounding halos. We explore these relationships down to halos of much lower mass (1011 h-1 M ) than previous studies. The halos are triaxial, with c/a ratios of 0.6±0.1 and a mean two-dimensional projected ellipticity of e=0.24. More massive halos are more flattened. The axis ratios rise out to 0.6rvir, beyond which they drop. The principal axes, in particular the minor axes, are very well aligned within 0.6 rvir. High-mass halos show particularly strong internal alignment. The angular momentum vectors are also reasonably well aligned except between the very outermost and very innermost regions of the halo. The angular momentum vectors tend to align with the minor axes, with a mean misalignment of 25°, and lie perpendicular to the major and intermediate axes. The properties of a halo at 0.4 rvir are quite characteristic of the properties at most other radii within the halo. There is a very strong tendency for the minor axes of halos to lie perpendicular to large-scale filaments, and a much weaker tendency for the major axes to lie along the filaments. This alignment extends to much larger separations for group and cluster mass halos than for galaxy mass halos. As a consequence, the intrinsic alignments of galaxies are likely weaker than previous predictions, which were based on the shapes of cluster mass halos. The angular momenta of the highest concentration halos tend to point toward other halos. The angular momenta of galaxy mass halos point parallel to filaments, while those of group and cluster mass halos show a very strong tendency to point perpendicular to the filaments. This suggests that group and cluster mass halos acquire most of their angular momentum from major mergers along filaments, while the accretion history of mass and angular momentum onto galaxy mass halos has been smoother.