The Origin of Spin-Alignment of Dark Matter Subhalos
Subhalo spin is essential for modeling galaxy formation and controlling systematic uncertainties in intrinsic alignment (IA) studies. However, the physical mechanisms governing subhalo spin acquisition within the tidal environments of host halos remain poorly understood. In this work, we investigate the alignment between subhalo and host halo spins using the high-resolution cosmological $N$-body simulation, Shin-Uchuu. We find that the spin alignment between subhalos and host halos becomes increasingly pronounced toward the central regions. Our analysis reveals that subhalos typically acquire spin in the same direction as their orbital angular momentum. Since the orbital angular momentum of most subhalos is aligned with the host halo spin, an overall alignment between subhalo and host spins emerges. When classified by orbital orientation, however, subhalo spins in the inner regions are found to be oriented perpendicularly or anti-parallel to the host spin for polar and retrograde orbits, respectively. These results provide strong evidence that subhalo spins are acquired through torques exerted by the tidal field of the host halo. Furthermore, we demonstrate that the mass ratio and the radial distance from the host center are the primary parameters governing subhalo spin alignment, while the dependence on the accretion redshift plays a less significant role compared to the radial distance and mass ratio.