Dynamical friction and measurements of the splashback radius in galaxy clusters

TL;DR

This study investigates the impact of dynamical friction on measuring the splashback radius in galaxy clusters, finding it affects smaller clusters but not massive ones, thus not explaining previous measurement discrepancies.

Contribution

The paper introduces idealized simulations controlling dynamical friction to assess its effect on splashback radius measurements in galaxy clusters.

Findings

Dynamical friction reduces splashback radius in smaller clusters.

No significant effect of dynamical friction on clusters with mass > 10^{14} M_sun.

Dynamical friction cannot fully explain measurement discrepancies.

Abstract

The splashback radius is one popular method of constraining the size of galaxy clusters, often measured through the logarithmic derivative of the galaxy number density profile. However, measuring the splashback radius through the galaxy number density has consistently produced smaller values of the splashback radius than those inferred from the underlying gravitational potential in simulations. Dynamical friction has been posited as one possible reason that splashback radii measured through galaxy number densities are reduced, since it decays the orbits of subhaloes within the halo. Dynamical friction is an emergent process, and as such, cannot be isolated or removed within N-body simulations. Here, we present simulations starting with isolated galaxy clusters drawn from the IllustrisTNG cosmological simulation, where we explicitly control dynamical friction through an idealized model. We show that although dynamical friction can reduce measurements of the splashback radius, it does not have a significant effect on clusters with $M_\mathrm{200,mean} > 10^{14} \mathrm{M_\odot}$, and thus cannot account for previously measured discrepancies.