# A detection of wobbling Brightest Cluster Galaxies within massive galaxy clusters

## TL;DR

This study detects wobbling of Brightest Cluster Galaxies in galaxy clusters, providing evidence for dark matter cores, using simulations and observations, challenging the assumption that BCGs are fixed at cluster centers.

## Contribution

It introduces a novel method to measure BCG wobbling, providing observational evidence for dark matter cores in galaxy clusters, which contrasts with standard cold dark matter predictions.

## Key findings

- Observed BCG wobble amplitude significantly exceeds CDM predictions.
- Detection of non-zero wobbling at 3σ confidence level.
- Supports the existence of dark matter cores in galaxy clusters.

## Abstract

A striking signal of dark matter beyond the standard model is the existence of cores in the centre of galaxy clusters. Recent simulations predict that a Brightest Cluster Galaxy (BCG) inside a cored galaxy cluster will exhibit residual wobbling due to previous major mergers, long after the relaxation of the overall cluster. This phenomenon is absent with standard cold dark matter where a cuspy density profile keeps a BCG tightly bound at the centre. We test this hypothesis using cosmological simulations and deep observations of 10 galaxy clusters acting as strong gravitational lenses. Modelling the BCG wobble as a simple harmonic oscillator, we measure the wobble amplitude, $A_w$, in the BAHAMAS suite of cosmological hydrodynamical simulations, finding an upper limit for the CDM paradigm of $A_w < 2$kpc at the 95% confidence limit. We carry out the same test on the data finding a non-zero amplitude of $A_w=11.82^{+7.3}_{-3.0}$kpc, with the observations dis-favouring $A_w = 0$ at the 3$\sigma$ confidence level. This detection of BCG wobbling is evidence for a dark matter core at the heart of galaxy clusters. It also shows that strong lensing models of clusters cannot assume that the BCG is exactly coincident with the large scale halo. While our small sample of galaxy clusters already indicates a non-zero $A_w$, with larger surveys, e.g. Euclid, we will be able to not only to confirm the effect but also to use it to determine whether or not the wobbling finds its origin in new fundamental physics or astrophysical process.