# Properties and Origin of Galaxy Velocity Bias in the Illustris Simulation

## TL;DR

This study uses the Illustris simulations to analyze galaxy velocity bias, revealing its dependence on galaxy-to-halo mass ratio and the dynamical history of satellites, with implications for modeling galaxy clustering.

## Contribution

It provides a detailed analysis of the origin and properties of galaxy velocity bias using hydrodynamical simulations, linking bias to galaxy mass, interactions, and accretion history.

## Key findings

- Galaxy velocity bias decreases with higher galaxy-to-halo mass ratio.
- Central galaxies are often not at rest relative to dark matter halos.
- Satellite galaxy bias depends on accretion time and distance from halo center.

## Abstract

We use the hydrodynamical galaxy formation simulations from the Illustris suite to study the origin and properties of galaxy velocity bias, i.e., the difference between the velocity distributions of galaxies and dark matter inside halos. We find that galaxy velocity bias is a decreasing function of the ratio of galaxy stellar mass to host halo mass. In general, central galaxies are not at rest with respect to dark matter halos or the core of halos, with a velocity dispersion above 0.04 times that of the dark matter. The central galaxy velocity bias is found to be mostly caused by the close interactions between the central and satellite galaxies. For satellite galaxies, the velocity bias is related to their dynamical and tidal evolution history after being accreted onto the host halos. It depends on the time after the accretion and their distances from the halo centers, with massive satellites generally moving more slowly than the dark matter. The results are in broad agreements with those inferred from modeling small-scale redshift-space galaxy clustering data, and the study can help improve models of redshift-space galaxy clustering.