Spin Alignment of Dark Matter Halos: Fast Halos

Abstract We investigate the spin alignment of dark matter halos by considering a mechanism somewhat similar to tidal locking; we dub it tidal-locking theory (TLT). While tidal torque theory (TTT) is responsible for the initial angular momentum of dark matter halos, TLT explains the angular momentum evolution during nonlinear ages. Our previous work showed that close encounters between halos could drastically change their angular momentum. This paper argues that TLT predicts partial alignment between the speed and spin direction for large high-speed halos. To examine this prediction, we use IllustrisTNG simulations and look for the alignment of the halos’ rotation axes. We find that the excess probability of alignment between spin and speed is about 10% at z = 0 for the large fast halos with velocities larger than twice the median. This spin–speed alignment weakens at z = 1 and disappears at z = 4. We also show that TTT predicts that the spin of a halo tends to be aligned with the middle eigendirection of the tidal tensor. Moreover, we find that the halos at z = 10 are preferentially aligned with the middle eigendirection of the tidal tensor with an excess probability of 15%. We show that TTT fails to predict the correct alignment at z = 0, while it works almost flawlessly at z = 10. These findings confirm that at earlier redshifts, during which mergers and fly-bys are rare, TTT works well, but after enough time, when fly-bys have occurred, the spin of the halos tends to align with speed for high-speed halos, due to the TLT effect.