Rings of Dark Matter in Collisions Between Clusters of Galaxies

TL;DR

This study uses N-body simulations to explore whether collision-induced dark matter rings can form in galaxy clusters, finding that such features are unlikely under realistic initial conditions, challenging previous interpretations of lensing data.

Contribution

The paper demonstrates through simulations that dark matter rings are unlikely to form from cluster collisions unless initial velocities are highly fine-tuned, questioning prior observational interpretations.

Findings

Simulations show a 'shoulder' in dark matter distribution post-collision.

No ring features appear unless initial velocities are perfectly circular.

Realistic velocity distributions from cosmological simulations do not produce rings.

Abstract

Several lines of evidence suggest that the galaxy cluster Cl0024+17, an apparently relaxed system, is actually a collision of two clusters, the interaction occurring along our line of sight. Recent lensing observations suggest the presence of a ring-like dark matter structure, which has been interpreted as the result of such a collision. In this paper we present $N$-body simulations of cluster collisions along the line of sight to investigate the detectability of such features. We use realistic dark matter density profiles as determined from cosmological simulations. Our simulations show a "shoulder" in the dark matter distribution after the collision, but no ring feature even when the initial particle velocity distribution is highly tangentially anisotropic ($\sigma_\theta/\sigma_r >> 1$). Only when the initial particle velocity distribution is circular do our simulations show such a feature. Even modestly anisotropic velocity distributions are inconsistent with the halo velocity distributions seen in cosmological simulations, and would require highly fine-tuned initial conditions. Our investigation leaves us without an explanation for the dark matter ring-like feature in Cl 0024+17 suggested by lensing observations.