Ming Li, Jun Pan, Liang Gao, Yipeng Jing, Xiaohu Yang, Xuebin Chi, Longlong Feng, Xi Kang, Weipeng Lin, Guihua Shang, Long Wang, Donghai Zhao, Pengjie Zhang
Analysis of the Pangu N-body simulation validates that bulk flow of halos follows Maxwellian distribution of which variance is consistent with prediction of linear perturbation theory of structure formation. We propose that consistency between observed bulk velocity and theories shall be examined at the effective scale as radius of spherical top-hat window function yielding the same smoothed velocity variance in linear theory as the sample window does. Then we compared some recently estimated bulk flows from observational samples with prediction of the $\Lambda$CDM model we used, some results deviate the expectation at level of $\sim 3\sigma$ but the tension is not as severe as previously claimed. We disclose that bulk flow is weakly correlated with dipole of internal mass distribution, alignment angle between mass dipole and bulk flow has broad distribution but is peaked at $\sim 30-50^\circ$, meanwhile bulk flow shows little dependence on mass of halos used for estimation. In the simulation of box size $1h^{-1}$Gpc, for a cell of radius $100^{-1}$Mpc the maximal bulk velocity is $>500\kms$, dipoles of environmental mass outside the cell are not tightly aligned with the bulk flow, instead are located randomly around it with separation angles $\sim 20-40^\circ$. In the cell showing largest bulk velocity there are slightly smaller number of low mass halos, however halos inside are clustered more strongly at scales $\gtrsim 20h^{-1}$Mpc, which might be a significant feature since the correlation between bulk flow and halo clustering actually grows into notable beyond such scales.
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http://arxiv.org/abs/1207.5338
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