Properties and Origin of Galaxy Velocity Bias in the Illustris Simulation

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.

Cosmological Evidence Modelling: a new simulation-based approach to constrain cosmology on non-linear scales

“…Tinker et al 2006;Reid et al 2014;Zhai et al 2019). We assume that centrals are located at the halo core which should greatly reduce the need for additional velocity offsets (Behroozi et al 2013;Guo et al 2016;Ye et al 2017). Similarly, satellite velocities are derived from solving the Jeans equation which allows physically motivated differences between the velocities of satellites and dark matter (Lange et al 2019a).…”

Maturing satellite kinematics into a competitive probe of the galaxy–halo connection

“…Throughout we assume that central galaxies reside at rest at the centre of their host haloes, which are assumed to be spherically symmetric. While it is known that centrals are not perfectly at rest relative to the centre-of-mass of their host halo, (van den Bosch et al 2005b;Behroozi et al 2013;Guo et al 2015aGuo et al ,b, 2016Ye et al 2017), this motion does not have a significant impact. The typical speed of a central is ∼ 20% of the root-mean-square speed of its satellites (Ye et al 2017).…”

“…Hence, some satellite galaxies will be on first infall, and their kinematics will not be in perfect virial equilibrium. As shown by Ye et al (2017) ignoring the resulting time-dependence in the Jeans equations (which are based on the assumption of a steady-state), may result in small biases in the inferred velocity dispersion. We address this issue in §8.3 by analysing mock catalogues where satellite galaxies are placed on subhaloes.…”

“…Furthermore, dark matter haloes are not fully relaxed. Thus, there is no a priori reason to assume that they would obey the Jeans equation (Ye et al 2017;Wang et al 2018). Additionally, we have neglected higher order moments of the velocity distribution and velocity anisotropy in our calculation.…”

Updated results on the galaxy–halo connection from satellite kinematics in SDSS

“…Central galaxies are always assumed to reside at the dark matter halo centre and to be at rest with the bulk velocity of the halo. While it is known that central galaxies have some residual velocity dispersion with respect to the halo (Behroozi et al 2013;Guo et al 2015a,b;Ye et al 2017), the dispersion is only of the order of 15% of that of the satellites. Thus, it will not strongly affect the total velocity dispersion between centrals and satellites (see Paper I for details).…”