Mass estimation in the outer non-equilibrium region of galaxy clusters

We discuss a new criterion to estimate the mass in the outer, non-equilibrium region of galaxy clusters, where the galaxy dynamics are dominated by an overall infall motion towards the cluster centre. In the framework of the spherical infall model, the local mean velocity of the infalling galaxies at every radius provides information about the integrated matter overdensity δ. Thus, a well-defined value of the overdensity δt is expected at the turnaround radius rt, that is the radius where the Hubble flow balances the infall motion. Within this scenario, we analysed the kinematical properties of a large catalogue of simulated clusters, using both dark matter particles and member galaxies as tracer of the infall motion. We also compared the simulation with analytical calculation performed in the spherical infall approximation, to analyse the dependence of the results on cosmology in spatially flat Universe. If we normalize cluster mass profiles by means of the turnaround mass Mt (i.e. the mass within rt), they are consistent with an exponential profile in the whole non-equilibrium region (0.5 ≲r/rt≲ 2). Turnaround radii are proportional to virialization radii (rt≃ 3.5rv), while turnaround masses are proportional to virialization masses, that is Mt≃ 1.7Mv, where Mv is the mass within rv. Actually, the mass evaluated within the turnaround radius is a more exhaustive evaluation of the total mass of the cluster. These results can be applied to the analysis of observed clusters.