Examining the Self-Interaction of Dark Matter through Central Cluster Galaxy Offsets

TL;DR

This study investigates the potential observable effects of dark matter self-interactions by measuring offsets between central galaxies and halo centers in galaxy clusters, finding results consistent with some level of dark matter self-interaction.

Contribution

It provides the first observational analysis of galaxy-halo offsets in relaxed clusters, offering evidence that supports the possibility of self-interacting dark matter.

Findings

Most clusters show significant offsets exceeding positional uncertainties.

The average offset is approximately 5.5 kpc in relaxed clusters.

Results are consistent with models of self-interacting dark matter.

Abstract

While collisionless cold dark matter models have been largely successful in explaining a wide range of observational data, some tensions still exist, and it remains possible that dark matter possesses a non-negligible level of self interactions. In this paper, we investigate a possible observable consequence of self-interacting dark matter: offsets between the central galaxy and the center of mass of its parent halo. We examine 23 relaxed galaxy clusters in a redshift range of 0.1 to 0.3 drawn from clusters in the Dark Energy Survey and the Sloan Digital Sky Survey which have archival Chandra X-ray data of sufficient depth for center and relaxation determination. We find that most clusters in our sample show non-zero offsets between the X-ray center, taken to be the centroid within the cluster core, and the central galaxy position. All of the measured offsets are larger, typically by an order of magnitude, than the uncertainty in the X-ray position due to Poisson noise. In all but six clusters, the measured offsets are also larger than the estimated, combined astrometric uncertainties in the X-ray and optical positions. A more conservative cut on concentration to select relaxed clusters marginally reduces but does not eliminate the observed offset. With our more conservative sample, we find an estimated mean X-ray to central galaxy offset of $\mu = 5.5 \pm 1.0$ kpc. Comparing to recent simulations, this distribution of offsets is consistent with some level of dark matter self interaction, though further simulation work is needed to place constraints.