Spin Alignment of Dark Matter Halos: Mad Halos

Abstract

We investigate the spin alignment of the dark matter halos by considering a mechanism somewhat similar to tidal locking. We dubbed it Tidal Locking Theory (TLT). While Tidal Torque Theory is responsible for the initial angular momentum of the dark matter halos, the Tidal locking Theory explains the angular momentum evolution during non-linear ages. Our previous work showed that close encounters between haloes could drastically change their angular momentum. The current manuscript argues that the tidal locking theory predicts partial alignment between speed and the spin direction for the large high-speed halos. To examine this prediction, we use the IllustrisTNG simulation and look for the alignment of the halos' rotation axis. We find that the excess probability of alignment between spin and speed is about 10 percent at $z=0$ for fast haloes; with velocities larger than twice the median. We show that tidal torque theory 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 percent. We show that tidal torque theory fails to predict correct alignment at $z=0$ while it works almost flawlessly at $z=10$.