Bulk flow of halos in \Lambda CDM simulation
  Analysis of the Pangu N-body simulation validates that the bulk flow of halos
follows a Maxwellian distribution which variance 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 \Lambda CDM model we used;
some results deviate from expectation at a level of ~ 3\sigma 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, the
alignment angle between the mass dipole and the bulk flow has a broad
distribution peaked at ~ 30-50 deg., 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 1Gpc/h, for a cell of radius 100 Mpc/h the maximal bulk velocity is
>500 km/s, 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 deg. In the fastest cell there is a slightly smaller
number of low-mass halos; however halos inside are clustered more strongly at
scales > ~ 20 Mpc/h, which might be a significant feature since the correlation
between bulk flow and halo clustering actually increases in significance beyond
such scales.
