Constraints on the Dark Matter Self-Interaction Cross-Section from the Merging Cluster 1E 0657-56

Abstract

We compare new maps of the hot gas, dark matter, and galaxies for 1E 0657-56, a cluster with a rare, high-velocity merger occurring nearly in the plane of the sky. The X-ray observations reveal a prominent bow shock and a bullet-like gas subcluster just exiting the collision site. The optical image shows that the gas bullet lags behind the subcluster galaxies; the weak-lensing mass map reveals a dark matter clump lying ahead of the collisional gas bullet, but coincident with the effectively collisionless galaxies. From these observations, one can directly constrain the cross-section of the dark matter self-interaction. That the dark matter is not fluid-like can be seen directly from the maps; more quantitative limits can be derived from four simple independent arguments. Our most sensitive constraint, σ/m<1 cm2 g−1, comes from the consistency of the subcluster mass-to-light ratio with the main cluster (and universal) value, which rules out a large mass loss due to dark matter particle collisions.