Turnaround radius of galaxy clusters in <i>N</i>-body simulations
Giorgos Korkidis, Vasiliki Pavlidou, Konstantinos Tassis, Evangelia Ntormousi, Theodore N. Tomaras, Konstantinos Kovlakas
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 R 200 . 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 M 200 > 10 13 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 R 11 – in a way that is analogous to defining the “virial” radius as R 200 – with the advantage that R 11 is shown in this work to correspond to a kinematically relevant scale in N -body simulations.