BULK FLOW OF HALOS IN ΛCDM SIMULATION

Analysis of the Pangu N-body simulation validates that the bulk flow of halos follows a Maxwellian distribution with variance that is consistent with the prediction of the linear theory of structure formation. We propose that the consistency between the observed bulk velocity and theories should be examined at the effective scale of the radius of a spherical top-hat window function yielding the same smoothed velocity variance in linear theory as the sample window function does. We compared some recently estimated bulk flows from observational samples with the prediction of the ΛCDM model we used; some results deviate from expectation at a level of ∼3σ, but the discrepancy is not as severe as previously claimed. We show that bulk flow is only weakly correlated with the dipole of the internal mass distribution, that the alignment angle between the mass dipole and the bulk flow has a broad distribution peaked at ∼30°–50°, and also that the bulk flow shows little dependence on the mass of the halos used in the estimation. In a simulation of box size 1 h−1 Gpc, for a cell of radius 100 h−1 Mpc the maximal bulk velocity is >500 km s−1; dipoles of the environmental mass outside the cell are not tightly aligned with the bulk flow, but are rather located randomly around it with separation angles ∼20°–40°. In the fastest cell there is a slightly smaller number of low-mass halos; however, halos inside are clustered more strongly at scales ≳ 20 h−1 Mpc, which might be a significant feature since the correlation between bulk flow and halo clustering actually increases in significance beyond such scales.