Turnaround radius of galaxy clusters in Nbody simulations
Abstract
We use Nbody simulations to examine whether a characteristic turnaround radius, as predicted from the spherical collapse model in a $\rm {\Lambda CDM}$ Universe, can be meaningfully identified for galaxy clusters, in the presence of full threedimensional effects. We use The Dark Sky Simulations and IllustrisTNG darkmatteronly 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. We find that: (a) Indeed, a single turnaround scale can meaningfully describe strongly nonspherical 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 Nbody simulations.
 Publication:

arXiv eprints
 Pub Date:
 December 2019
 arXiv:
 arXiv:1912.08216
 Bibcode:
 2019arXiv191208216K
 Keywords:

 Astrophysics  Cosmology and Nongalactic Astrophysics
 EPrint:
 Submitted to A&