The behaviour of dark matter associated with 4 bright cluster galaxies in the 10kpc core of Abell 3827
by Richard Massey, Liliya Williams, Renske Smit, et al.
Monthly Notices of the Royal Astronomical Society, June 2015, 449(4):3393-3406. doi: 10.1093/mnras/stv467, First published online April 14, 2015, http://mnras.oxfordjournals.org/content/449/4/3393

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 distri-bution is very rare, and suggests recent formation by everal simultaneous mergers. We map the distribution of associated dark matter, using new Hubble Space Telescope imaging and VLT/MUSE integral field spectroscopy of a gravitational lens system threaded through the cluster core. Analysis of ground-based imaging previously sug-gested that dark matter associated with one galaxy had become offset by up to 6kpc from its stars – perhaps lagging behind during its long infall because of a drag force created by dark matter self-interactions. Exploiting our new, high-resolution data, we find that each of the central galaxies retains an associated massive halo. At least one of the galaxies is offset from its dark matter. The best-constrained position is 1.62 +0.50/−0.47 kpc from the stars, where the 68% confidence limit includes both statistical error and systematic biases in mass modelling. With such a small physical offset, 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 σDM/m ~ (1.7 ± 0.7)×10⁻⁴ cm²/g.