A possible explanation of vanishing halo velocity bias

TL;DR

This paper explains the observed near-unity halo velocity bias by extending the peak model to non-Gaussian density fields at low redshift, aligning theoretical predictions with recent simulation results.

Contribution

It introduces an extension of the peak model to non-Gaussian density fields, providing a better explanation for the vanishing halo velocity bias observed in simulations.

Findings

Predicted velocity bias deviation is weaker than previous models.

Analytical expressions match simulation results more closely.

Extension of the peak model improves understanding of halo velocity bias.

Abstract

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.