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

## TL;DR

This paper revises the upper limits on dark matter self-interaction cross sections derived from galaxy cluster mergers, highlighting errors in previous measurements and discussing the limitations of current methods.

## Contribution

It identifies significant errors in previous offset measurements, corrects the upper limit on dark matter self-interaction, and discusses the challenges of using merger data to constrain dark matter properties.

## Key findings

- Revised upper limit on dark matter cross section to ~2 cm^2/g
- Errors in previous measurements significantly affected limits
- Single-band imaging methods have inherent biases

## Abstract

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. Although this particular sample does not tightly constrain self-interacting dark matter models when analyzed this way, we discuss how merger ensembles may be used more effectively in the future. We conclude that errors inherent in using single-band imaging to identify mass and light peaks do not necessarily average out in a sample of this size, particularly when a handful of substructures constitute a majority of the weight in the ensemble.