Cosmological tidal shear

The effect of cosmological tidal shear on the structure and kinematics of dark halos is examined using N-body simulations of the collapse of density peaks in the cold dark matter model. The origin of the triaxial nature of dark halos is investigated by comparing the shapes of the density peaks to the shapes of dark halos. Dark halos are generally flatter than their progenitor density perturbations. The shape distribution of dark halos depends strongly on the strength and orientation of the tidal shear field. The kinematics are studied using measurements of angular momentum, streaming motions, tumbling, and velocity ellipsoids of the dark halos. Most of the angular momentum is in the form of streaming motions with tumbling motions having periods exceeding 4 Gyr. The angular momentum vector tends to align with the minor axis of the triaxial dark halos. The alignment of the angular momentum vector probably originates from the alignment of the linear regime torque acting on a triaxial density perturbation. The origin of the tendency for the small misalignment angle between the rotation vector and the apparent minor axis in elliptical galaxies is probably due to this effect. Tidal torques are found to isotropize the velocity ellipsoids of dark halos at large radii in some cases.