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 (10 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 0.6 rvir, 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, though with more scatter. The angular momentum vectors tend to align with the minor axes 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. 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 the accretion of subhalos along filaments, while the angular momentum in galaxy mass halos has retained some memory of the primordial tidal torques that acted on the halo during its formation. Subject headings: galaxies: formation — galaxies: halos — galaxies: structure — dark matter — galaxies: clusters: general — methods: N-body simulations