A Possible Explanation of Vanishing Halo Velocity Bias

Recently Chen et al. (2018, ApJ, 861, 58) accurately determined the volume weighted halo velocity bias in simulations, and found that the deviation of velocity bias from unity is much weaker than the peak model prediction. Here we present a possible explanation of this vanishing velocity bias. The starting point is that, halos are peaks in the low redshift {\it non-Gaussian} density field with smoothing scale $R_{\Delta}$ (virial radius), instead of peaks in the high redshift initial {\it Gaussian} density field with a factor of $\mathcal{O}(\Delta^{1/3})$ larger smoothing scale. Based on the approximation that the density field can be Gaussianized by a local and monotonic transformation, we extend the peak model to the non-Gaussian density field and derive the analytical expression of velocity dispersion and velocity power spectrum of these halos. The predicted deviation of velocity bias from unity is indeed much weaker than the previous prediction, and the agreement with the simulation results is significantly improved.