Protruding bullet heads indicating dark matter pull

TL;DR

This paper proposes that protruding bullet head features in galaxy cluster X-ray images are caused by dark matter's gravitational pull during mergers, offering a new way to detect dark matter.

Contribution

It introduces a novel explanation for bullet morphology in galaxy clusters, linking it to dark matter effects observable via X-ray imaging.

Findings

Protruding bullet heads are caused by dark matter gravitational pull.

The effect is common in major galaxy mergers.

Distinct bullet morphologies are consistent with observations.

Abstract

A clump moving through the intracluster medium of a galaxy cluster can drive a bow shock trailed by a bullet-like core. In some cases, such as in the prototypical Bullet cluster, X-rays show a gas bullet with a protruding head and pronounced shoulders. We point out that these features, while difficult to explain without dark matter (DM), naturally arise as the head of the slowed-down gas is gravitationally pulled forward toward its unhindered DM counterpart. X-ray imaging thus provides a unique, robust probe of the offset, collisionless DM, even without gravitational lensing or other auxiliary data. Numerical simulations and a toy model suggest that the effect is common in major mergers, is often associated with a small bullet-head radius of curvature, and may lead to distinct bullet morphologies, consistent with observations.