The Mismeasure of Mergers: Revised Limits on Self-interacting Dark Matter in Merging Galaxy Clusters

Understand

In an influential recent paper, Harvey et al (2015) derive an upper limit to the self-interaction cross section of dark matter ($\sigma_{\rm DM} < 0.47$ cm$^2$/g at 95\% confidence) by averaging the dark matter-galaxy offsets in a sample of merging galaxy clusters.

• Using much more comprehensive data on the same clusters, we identify several substantial errors in their offset measurements.
• Correcting these errors relaxes the upper limit on $\sigma_{\rm DM}$ to $\lesssim 2$ cm$^2$/g, following the Harvey et al prescription for relating offsets to cross sections in a simple solid body scattering model.
• Furthermore, many clusters in the sample violate the assumptions behind this prescription, so even this revised upper limit should be used with caution.