# Observable tests of self-interacting dark matter in galaxy clusters: BCG wobbles in a constant density core

## TL;DR

This study compares observations of Brightest Cluster Galaxy (BCG) offsets in galaxy clusters with simulations to test for self-interacting dark matter, finding current data slightly favors cold dark matter but constrains SIDM cross-sections.

## Contribution

It provides the first comparison of observed BCG offsets with cosmological simulations including SIDM, constraining the dark matter self-interaction cross-section.

## Key findings

- Observed BCG offsets are consistent with CDM at 1.5σ.
- Constraints on SIDM cross-section are σ/m < 0.12 cm²/g at 68%.
- Larger surveys will tighten these constraints.

## Abstract

Models of Cold Dark Matter predict that the distribution of dark matter in galaxy clusters should be cuspy, centrally concentrated. Constant density cores would be strong evidence for beyond-CDM physics, such as Self-Interacting Dark Matter (SIDM). An observable consequence would be oscillations of the Brightest Cluster Galaxy (BCG) in otherwise relaxed galaxy clusters. Offset BCGs have indeed been observed - but only interpreted via a simplified, analytic model of oscillations. We compare these observations to the BAHAMAS-SIDM suite of cosmological simulations, which include SIDM and a fully hydrodynamical treatment of star formation and feedback. We predict that the median offset of BCGs increases with the SIDM cross-section, cluster mass and the amount of stellar mass within 10kpc, while CDM exhibits no trend in mass. Interpolating between the simulated cross-sections, we find that the observations (of 10 clusters) is consistent with CDM at the ~1.5$\sigma$ level, and prefer cross-section $\sigma$/m < 0.12(0.39)cm$^2$/g at 68% (95%) confidence level. This is on the verge of ruling out velocity-independent dark matter self-interactions as the solution to discrepancies between the predicted and observed behaviour of dwarf galaxies, and will be improved by larger surveys by Euclid or SuperBIT.