The impact of environment on the dynamical structure of satellite systems

Abstract

We examine the effects of environment on the dynamical structure of satellite systems based on the Millennium--II Simulation. Satellite halos are defined as sub--halos within the virial radius of a host halo. The satellite sample is restricted to those sub--halos which showed a maximum circular velocity above 30 km/s at the time of accretion. Host halo masses range from 10^11 to 10^14 Msol/h. We compute the satellites' average accretion redshift, z_acc, velocity dispersion, sigma, and velocity anisotropy parameter, beta, utilising stacked satellite samples of equal mass hosts at similar background densities. The main results are: (1) On average satellites within hosts in high density environments are accreted earlier (Delta z~ 0.1$) compared to their counterparts at low densities. For host masses above 5 times10^13 Msol/h this trend weakens and may reverse for higher host masses; (2) The velocity dispersion of satellites in low density environments follows that of the host, i.e. no velocity bias is observed for host halos at low densities independent of host mass. However, for low mass hosts in high density environments the velocity dispersion of the satellites can be up to ~30% larger than that of the host halo, i.e. the satellites are dynamically hotter than their host halos. (3) The anisotropy parameter depends on host mass and environment. Satellites of massive hosts show more radially biased velocity distributions. Moreover in low density environments satellites have more radially biased velocities (Delta beta > 0.1) compared to their counterparts in high density environments. We believe that our approach allows to predict a similar behaviour for observed satellite galaxy systems.