The behaviour of dark matter associated with 4 bright cluster galaxies in the 10kpc core of Abell 3827

TL;DR

This study maps dark matter in a galaxy cluster core, revealing an offset in dark matter distribution that could suggest self-interacting dark matter, with implications for understanding dark matter properties.

Contribution

It provides the first direct measurement of dark matter offsets in a cluster core, supporting the possibility of dark matter self-interactions affecting galaxy dynamics.

Findings

Detected a 1.62+/-0.48 kpc dark matter offset in one galaxy.

Offsets are consistent with predictions from self-interacting dark matter models.

Results suggest dark matter self-interaction cross-section of approximately 1.7x10^{-4} cm^2/g.

Abstract

Galaxy cluster Abell 3827 hosts the stellar remnants of four almost equally bright elliptical galaxies within a core of radius 10kpc. Such corrugation of the stellar distribution is very rare, and suggests recent formation by several simultaneous mergers. We map the distribution of associated dark matter, using new Hubble Space Telescope imaging and VLT/MUSE integral field spectroscopy of a gravitationally lensed system threaded through the cluster core. We find that each of the central galaxies retains a dark matter halo, but that (at least) one of these is spatially offset from its stars. The best-constrained offset is 1.62+/-0.48kpc, where the 68% confidence limit includes both statistical error and systematic biases in mass modelling. Such offsets are not seen in field galaxies, but are predicted during the long infall to a cluster, if dark matter self-interactions generate an extra drag force. With such a small physical separation, it is difficult to definitively rule out astrophysical effects operating exclusively in dense cluster core environments - but if interpreted solely as evidence for self-interacting dark matter, this offset implies a cross-section sigma/m=(1.7+/-0.7)x10^{-4}cm^2/g x (t/10^9yrs)^{-2}, where t is the infall duration.