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

TL;DR

This paper introduces a new method to estimate galaxy cluster masses in the outer non-equilibrium regions using infall velocities, validated with simulations and analytical models, improving mass measurement accuracy.

Contribution

The study presents a novel criterion based on infall motion and overdensity at the turnaround radius to estimate cluster mass, supported by simulations and analytical calculations.

Findings

Mass profiles normalized by turnaround mass follow an exponential profile.

Turnaround radius is approximately 3.5 times the virial radius.

Turnaround mass is about 1.7 times the virial mass.

Abstract

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.