Dynamical friction and measurements of the splashback radius in galaxy clusters

TL;DR

The paper investigates whether dynamical friction (DF) can explain why splashback radii measured from galaxy number densities are smaller than those inferred from the underlying potential. By seed­ing clusters with IllustrisTNG subhaloes and evolving orbits in a static spherical potential with and without an analytical DF term (using Chandrasekhar’s formula), the authors isolate the DF effect on the splashback radius. They find that DF can reduce the measured $R_{ ext{sp}}$ by up to ~10%, but in massive clusters with $M_{200, ext{mean}} > 10^{14}\,M_\odot$ the effect is small, and DF alone cannot account for the observed discrepancies. The work also highlights that DF’s impact grows with the subhalo–host mass fraction and residence time, but remains subdominant compared to other potential factors such as selection biases and baryonic processes. Overall, DF is unlikely to be the sole explanation for the observed differences in splashback radii, motivating further studies with more complete physics and growth history.

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.

Figures (13)

• Figure 1: Shows selected galaxy orbits, evolved for 10 Gyr in a cluster. The background colour shows the cluster's spherically averaged dark matter density. Orbits for each galaxy are shown with and without dynamical friction enabled; those with dynamical friction are shown by brighter and thinner lines. The transparency of a line at a given position depends on its distance into the page. Selected galaxy masses are labeled, with solid and dashed lines connecting the labels to the final positions of the galaxies with and without dynamical friction, respectively.
• Figure 2: Time evolution of average galaxy number density profiles (upper) and differential density profiles (lower) through 10 Gyr of evolution, as shown in the colourbar. The profiles are evolved with DF disabled (on left) and enabled (on right). Haloes are stacked with $M_{200, \mathrm{mean}}$ between $10^{14}$ and $10^{14.5}~\mathrm{M}_{\odot}$ (185 haloes), and include all subhaloes.
• Figure 3: Bootstrapped, unfitted differential density profiles for haloes of $M_{200, \mathrm{mean}}$ between $10^{13}$ and $10^{13.5}~\mathrm{M}_{\odot}$ evolved for 5 Gyr, among subhaloes with minimum stellar mass cuts, as labeled in each panel. Profiles evolved without DF are shown in blue, and those with DF are shown in pink. Vertical dashed lines mark the respective median splashback radius, in matching colours.
• Figure 4: Splashback radius distributions after 5 Gyr for haloes with masses of $10^{13}~<~M_{200, \mathrm{mean}}/\mathrm{M}_{\odot}~<~10^{13.5}$. Minimum stellar mass cuts are shown in different colors. Distributions without dynamical friction are marked with dashed lines, while those evolved with dynamical friction use solid lines.
• Figure 5: Splashback radius distributions after 5 Gyr from bootstrapped and fitted profiles, divided by halo mass groups of 0.5-dex (separated by vertical black dashed lines.) Within each halo mass division, horizontal position is unconnected to mass. Minimum subhalo stellar mass cuts are individually coloured. Median splashback radii calculated from haloes evolved without DF are shown with unfilled square markers, while those evolved with DF are shown with filled squares. The error bars show the 16-84 percentile of the range of each distribution.