Velocity distributions in clusters of galaxies

We demonstrate, using a dissipationless N-body simulation of a galaxy cluster, that the velocity dispersion of subhalos depends strongly on the selection criterion: Applying a lower limit on the present bound mass of subhalos leads to higher velocity dispersions for the subhalos compared to the diffuse dark matter (positive velocity bias). Hardly any bias is detected if subhalos are required to exceed a minimal mass before accretion onto the host. It has been shown, that the latter criterion results in subhalo samples that closely resemble the spatial distribution of observed galaxy clusters, therefore it is referred to as galaxy sample. The velocity distributions of both, the galaxy sample and the diffuse dark matter follow Maxwell distributions with very similar temperatures. The velocity dispersion of the diffuse dark matter is translated into a temperature profile of the intra cluster gas (ICM) which is in good agreement with recent X-ray observations. From the temperature the adiabatic sound speed can be derived. A comparison of this sound speed with the velocities of galaxies results in an average Mach number of 1.24. 65% of the galaxies move supersonically and 8% have Mach numbers larger than 2