Light/Mass Offsets in the Lensing Cluster Abell 3827: Evidence for Collisional Dark Matter?

TL;DR

This study investigates potential dark matter self-interactions by analyzing light/mass offsets in the galaxy cluster Abell 3827, finding evidence suggestive of collisional dark matter with specific cross-section constraints.

Contribution

The paper presents the first evidence of a dark matter offset in Abell 3827, constraining dark matter self-interaction cross-section using free-form lens reconstruction.

Findings

Detection of a 6 kpc dark matter offset from a galaxy.

Constraints on dark matter self-interaction cross-section: ~4.5×10^{-7} cm^2/g.

Robustness confirmed through multiple reconstructions and synthetic tests.

Abstract

If dark matter has a non-zero self-interaction cross-section, then dark matter halos of individual galaxies in cluster cores should experience a drag force from the ambient dark matter of the cluster, which will not affect the stellar components of galaxies, and thus will lead to a separation between the stellar and dark matter. If the cross-section is only a few decades below its current astrophysically determined upper limit, then kpc-scale separations should result. However, such separations will be observable only under very favorable conditions. Abell 3827 is a nearby late stage cluster merger with four massive central ellipticals within 20 kpc of each other. The ten strong lensing images tightly surrounding the ellipticals provide an excellent set of constraints for a free-form lens reconstruction. Our free-form mass maps show a massive dark extended clump, about 6 kpc from one of the ellipticals. The robustness of this result has been tested with many reconstructions, and confirmed with experiments using synthetic lens mass distributions. Interpreted in terms of dark matter collisionality, our result yields sigma/m ~ 4.5 10^{-7} (t/10^{10} yr)^{-2} cm^2/g, where t is the merger's age.