The Dynamical Equilibrium of Galaxy Clusters

If a galaxy cluster is effectively in dynamical equilibrium then all galaxy populations within the cluster must have distributions in velocity and position that individually reflect the same underlying mass distribution, although the derived virial masses can be quite different. Specifically, within the CNOC cluster sample the virial radius of the red galaxy population is, on the average, a factor of $2.05 \pm 0.34$ smaller than that of the blue population. The red galaxies also have a smaller RMS velocity dispersion, a factor of $1.31 \pm 0.13$ within our sample. Consequently, the virial mass calculated from the blue galaxies is $3.5 \pm 1.3$ times larger than from the red galaxies. However, applying the Jeans equation of stellar-hydrodynamical equilibrium to the red and blue subsamples separately give statistically identical cluster mass profiles. This is strong evidence that these clusters are effectively equilibrium systems, and therefore empirically demonstrates that the masses in the virialized region are reliably estimated using dynamical techniques.