We discuss a new criterion to estimate the mass in the outer , non-equilibrium region of galaxy clusters , where the galaxy dynamics is 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 \delta .
Thus , a well-defined value of the overdensity \delta _ { t } is expected at the turnaround radius r _ { t } , i.e .
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 analyze the dependence of the results on cosmology in spatially flat universe .
If we normalize cluster mass profiles by means of the turnaround mass M _ { t } ( i.e .
the mass within r _ { t } ) , they are consistent with an exponential profile in the whole non-equilibrium region ( 0.5 \la r / r _ { t } \la 2 ) .
Turnaround radii are proportional to virialization radii ( r _ { t } \simeq 3.5 r _ { v } ) , while turnaround masses are proportional to virialization masses , i.e . M _ { t } \simeq 1.7 M _ { v } , where M _ { v } is the mass within r _ { v } .
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 .