A random walk model for the evolution of the halo spin vector

ABSTRACT We follow the spin vector evolutions of well resolved dark matter haloes (containing more than 300 particles) in merger tree main branches from the Millennium and Millennium-II N-body simulations, from $z\sim 3.3$ to $z=0$. We find that there seems to be a characteristic plane for the spin vector evolution along each main branch. In the direction perpendicular to it, spin vectors oscillate around the plane, while within the plane, spin vectors show a coherent direction change as well as a diffusion in direction (possibly corresponds to a Gaussian white noise). This plane may reflect the geometry of surrounding large-scale structures. We also construct a simple stochastic model in which halo spin vector evolution is assumed to be driven by accretion of halo mass and angular momentum. This model can reproduce major features of the results from N-body simulations.