Radial Alignment in Simulated Clusters

TL;DR

This study uses high-resolution simulations to demonstrate that dark matter substructures in galaxy clusters tend to align radially with their hosts, likely due to tidal torquing effects during orbital passages.

Contribution

It provides the first detailed simulation-based evidence of persistent radial alignment of subhalos, suggesting tidal torquing as the underlying physical mechanism.

Findings

Subhalos show a strong radial alignment with host halos.

Alignment peaks after the first virial radius crossing.

Alignment persists through multiple pericentric passages.

Abstract

Observational evidence for the radial alignment of satellites with their dark matter host has been accumulating steadily in the past few years. The effect is seen over a wide range of scales, from massive clusters of galaxies down to galaxy-sized systems, yet the underlying physical mechanism has still not been established. To this end, we have carried out a detailed analysis of the shapes and orientations of dark matter substructures in high-resolution N-body cosmological simulations. We find a strong tendency for radial alignment of the substructure with its host halo: the distribution of halo major axes is very anisotropic, with the majority pointing towards the center of mass of the host. The alignment peaks once the sub-halo has passed the virial radius of the host for the first time, but is not subsequently diluted, even after the halos have gone through as many as four pericentric passages. This evidence points to the existence of a very rapid dynamical mechanism acting on these systems and we argue that tidal torquing throughout their orbits is the most likely candidate.