Evidence for Dark Matter Self-Interactions via Collisionless Shocks in Cluster Mergers

TL;DR

This paper explores how collective plasma effects in charged dark matter can produce collisionless shocks in galaxy cluster mergers, offering new insights into dark matter self-interactions beyond two-body scatterings.

Contribution

It introduces a model of charged dark matter with plasma effects causing collisionless shocks, providing a novel mechanism for dark matter self-interactions in galaxy clusters.

Findings

Dark plasma effects can produce observable collisionless shocks in cluster mergers.

Charged dark matter can be separated from stars and non-interacting halos during collisions.

The model alleviates the missing satellite problem and avoids other astrophysical constraints.

Abstract

While dark matter self-interactions may solve several problems with structure formation, so far only the effects of two-body scatterings of dark matter particles have been considered. We show that, if a subdominant component of dark matter is charged under an unbroken $U(1)$ gauge group, collective dark plasma effects need to be taken into account to understand its dynamics. Plasma instabilities can lead to collisionless dark matter shocks in galaxy cluster mergers which might have been already observed in the Abell 3827 and 520 clusters. As a concrete model we propose a thermally produced dark pair plasma of vectorlike fermions. In this scenario the interacting dark matter component is expected to be separated from the stars and the non-interacting dark matter halos in cluster collisions. In addition, the missing satellite problem is softened, while constraints from all other astrophysical and cosmological observations are avoided.