Colliding clusters and dark matter self-interactions

TL;DR

This paper investigates how different types of dark matter self-interactions affect the spatial offset between dark matter halos and galaxies during collisions, finding that observable separation is limited but can reveal interaction properties.

Contribution

It distinguishes the effects of frequent small-momentum and rare large-momentum transfer self-interactions on halo-galaxy offsets, providing insights into dark matter properties.

Findings

Neither effect causes complete separation under conservative bounds.

Offsets are largest shortly after collision but not in evolved systems.

The fraction of large momentum transfer collisions can inform dark matter nature.

Abstract

When a dark matter halo moves through a background of dark matter particles, self-interactions can lead to both deceleration and evaporation of the halo and thus shift its centroid relative to the collisionless stars and galaxies. We study the magnitude and time evolution of this shift for two classes of dark matter self-interactions, viz. frequent self-interactions with small momentum transfer (e.g. due to long-range interactions) and rare self-interactions with large momentum transfer (e.g. contact interactions), and find important differences between the two cases. We find that neither effect can be strong enough to completely separate the dark matter halo from the galaxies, if we impose conservative bounds on the self-interaction cross-section. The majority of both populations remain bound to the same gravitational potential and the peaks of their distributions are therefore always coincident. Consequently any apparent separation is mainly due to particles which are leaving the gravitational potential, so will be largest shortly after the collision but not observable in evolved systems. Nevertheless the fraction of collisions with large momentum transfer is an important characteristic of self-interactions, which can potentially be extracted from observational data and provide an important clue as to the nature of dark matter.