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