Splashback radius of nonspherical dark matter halos from cosmic density and velocity fields

Abstract: We investigate the splashback features of dark-matter halos based on cosmic density and velocity fields. Besides the density correlation function binned by the halo orientation angle which was used in the literature, we introduce, for the first time, the corresponding velocity statistic, alignment momentum correlation function, to take into account the asphericity of halos. Using large-volume, high-resolution N-body simulations, we measure the alignment statistics of density and velocity. On halo scales, $x\sim R_\mathrm{200m} \sim 1Mpc/h$, we detect a sharp steepening in the momentum correlation associated with the physical halo boundary, or the splashback feature, which is found more prominent than in the density correlation. We also find that the splashback radius determined from the density correlation becomes $\sim 3.5\%$ smaller than that from the momentum correlation, with their correlation coefficient being 0.605. Moreover, the orientation-dependent splashback feature due to halo asphericity is measured when the density profile is determined by dark-matter particles, which can be used as a test of collisional CDM since the halo shape is predicted to be rounder in such a model.