Turnaround radius of galaxy clusters in N-body simulations

¹ Department of Physics and Institute for Theoretical and Computational Physics, University of Crete, 70013 Heraklio, Greece
e-mail: gkorkidis@physics.uoc.gr, pavlidou@physics.uoc.gr
² Institute of Astrophysics, Foundation for Research and Technology – Hellas, Vassilika Vouton, 70013 Heraklio, Greece
³ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy

Received: 17 December 2019
Accepted: 27 April 2020

Abstract

Aims. We use N-body simulations to examine whether a characteristic turnaround radius, as predicted from the spherical collapse model in a ΛCDM Universe, can be meaningfully identified for galaxy clusters in the presence of full three-dimensional effects.

Methods. We use The Dark Sky Simulations and Illustris-TNG dark-matter-only cosmological runs to calculate radial velocity profiles around collapsed structures, extending out to many times the virial radius R₂₀₀. There, the turnaround radius can be unambiguously identified as the largest nonexpanding scale around a center of gravity.

Results. We find that: (a) a single turnaround scale can meaningfully describe strongly nonspherical structures. (b) For halos of masses M₂₀₀ >  10¹³ M_⊙, the turnaround radius R_ta scales with the enclosed mass M_ta as M_ta^1/3, as predicted by the spherical collapse model. (c) The deviation of R_ta in simulated halos from the spherical collapse model prediction is relatively insensitive to halo asphericity. Rather, it is sensitive to the tidal forces due to massive neighbors when these are present. (d) Halos exhibit a characteristic average density within the turnaround scale. This characteristic density is dependent on cosmology and redshift. For the present cosmic epoch and for concordance cosmological parameters (Ω_m ∼ 0.3; Ω_Λ ∼ 0.7) turnaround structures exhibit a density contrast with the matter density of the background Universe of δ ∼ 11. Thus, R_ta is equivalent to R₁₁ – in a way that is analogous to defining the “virial” radius as R₂₀₀ – with the advantage that R₁₁ is shown in this work to correspond to a kinematically relevant scale in N-body simulations.