The Dynamical Equilibrium of Galaxy Clusters

If a galaxy cluster is effectively in dynamical equilibrium then all galaxypopulations within the cluster must have distributions in velocity and positionthat individually reflect the same underlying mass distribution, although thederived virial masses can be quite different. Specifically, within the CNOCcluster sample the virial radius of the red galaxy population is, on theaverage, 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 theblue galaxies is $3.5 \pm 1.3$ times larger than from the red galaxies.However, applying the Jeans equation of stellar-hydrodynamical equilibrium tothe red and blue subsamples separately give statistically identical clustermass profiles. This is strong evidence that these clusters are effectivelyequilibrium systems, and therefore empirically demonstrates that the masses inthe virialized region are reliably estimated using dynamical techniques.