On the dynamical state of galaxy clusters: insights from cosmological simulations - II.

Using a suite of cosmology simulations of a sample of >120 galaxy clusters with log (M-DM,M- vir) <= 14.5. We compare clusters that form in purely dark matter (DM) run and their counterparts in hydro-runs and investigate four independent parameters that are normally used to classify dynamical state. We find that the virial ratio eta in hydro-dynamical runs is similar to 10 per cent lower than in the DM run, and there is no clear separation between the relaxed and unrelaxed clusters for any parameter. Further, using the velocity dispersion deviation parameter zeta, which is defined as the ratio between cluster velocity dispersion sigma and the theoretical prediction sigma(t) = root GM(total)/R, we find that there is a linear correlation between the virial ratio eta and this zeta parameter. We propose to use this zeta parameter, which can be easily derived from observed galaxy clusters, as a substitute of the eta parameter to quantify the cluster dynamical state.