Intrinsic Alignments and Splashback Radius of Dark Matter Halos from Cosmic Density and Velocity Fields

We investigate the effects of intrinsic alignments (IA) of dark-matter halo shapes on cosmic density and velocity fields from cluster to cosmic scales beyond 100 Mpc/h. 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 two velocity statistics, the angle-binned pairwise infall momentum and momentum correlation function. Using large-volume, high-resolution N-body simulations, we measure the alignment statistics of density and velocity, both in real and redshift space. We find that the alignment signal is not amplified by redshift-space distortions at linear scales. Behaviors of IA in the velocity statistics are similar to those in the density statistics, except that the halo orientations are aligned with the velocity field up to a scale larger than those with the density field, x>100 Mpc/h. On halo scales, x~ R_{200m} ~ 1 Mpc/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. Our results indicate that observations of IA with the velocity field can provide additional information on cosmological models from large scales and on physical sizes of halos from small scales.