The Number Excess of Misaligned Interactions in the Spin-Orbit Alignment of Interacting Halo Pairs

Using cosmological N-body simulations, we investigate the spin-orbit alignment (SOA) between the spin of a halo (S) and the orbital angular momentum of its interacting neighbor (L). We identify interacting halo pairs with mass ratios from 1/3 to 3 and classify them into the various subsamples with respect to the halo mass, environment, and total energy of the pair system. In the probability distribution of angle ($\theta_{\rm SL}$) between the halo spin and the orbital angular momentum, we find the strong prograde alignment in that the number fractions of neighbors on the prograde orbit (0<$\theta_{\rm SL}$<90) are ~75% and ~60% for mergers and flybys, respectively. The prograde fraction decreases with the increasing halo mass and large-scale density. We also find a number excess of prograde-polar interactions (60<$\theta_{\rm SL}$<90) in the SOA distribution, which tends to be higher both for more massive halos and in less dense environments. The prograde-polar interaction implies the presence of misaligned orbital angular momentum, which can tilt the halo spin. We discuss the physical origin of the SOA.