Alignments of interacting haloes in the Horizon run 4 simulation

Interactions such as mergers and flybys play a fundamental role in shaping galaxy morphology. We used the Horizon Run 4 cosmological N-body simulations to study the aligments of spins and shapes of interacting haloes as a function of the halo mass and large-scale density. Interactions preferentially occur in the plane of rotation, and in the direction of the major axis of prolate haloes, and the trajectories are preferentially radial and prograde. We found a very strong alignment of the shapes already at redshift as high as 4. The spins are initially unaligned or even anti-aligned, and become more and more aligned as the redshift decreases. The alignment signals are stronger and evolve more at lower densities, and mass plays a secondary role.