Turnaround radius of galaxy clusters in N-body simulations

Abstract

We use N-body simulations to examine whether a characteristic turnaround radius, as predicted from the spherical collapse model in a $\mathrm{\Lambda CDM}$ Universe, can be meaningfully identified for galaxy clusters, in the presence of full three-dimensional effects. 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 non-expanding scale around a center of gravity. We find that: (a) Indeed, a single turnaround scale can meaningfully describe strongly non-spherical structures. (b) For halos of masses $M_{200}>10^{13}{M}_{\odot }$, 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 insensitive to halo asphericity. Rather, it is sensitive to the tidal forces due to massive neighbors when such 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 (${\Omega }_m\sim 0.7$; ${\Omega }_{\Lambda }\sim 0.3$) turnaround structures exhibit an average matter density contrast with the background Universe of $\delta \sim 11$. Thus $R_{ta}$ is equivalent to $R_{11}$ -- in a way 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.

Submitted to A&A, nine pages, 10 figures

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• Rotation-Dependent Catastrophic Disruption of Gravitational Aggregates
• Splashback in accreting dark matter halos
• The virialized mass of dark matter haloes
• Turning Around along the Cosmic Web
• Turnaround size of non-spherical structures
• Turnaround density as a probe of the cosmological constant

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