Mass-Galaxy offsets in Abell 3827, 2218 and 1689: intrinsic properties
  or line-of-sight substructures?
  We have made mass maps of three strong-lensing clusters, Abell 3827, Abell
2218 and Abell 1689, in order to test for mass-light offsets. The technique
used is GRALE, which enables lens reconstruction with minimal assumptions, and
specifically with no information about the cluster light being given. In the
first two of these clusters, we find local mass peaks in the central regions
that are displaced from the nearby galaxies by a few to several kpc. These
offsets {\em could\/} be due to line of sight structure unrelated to the
clusters, but that is very unlikely, given the typical levels of chance
line-of-sight coincidences in $\Lambda CDM$ simulations --- for Abell 3827 and
Abell 2218 the offsets appear to be intrinsic. In the case of Abell 1689, we
see no significant offsets in the central region, but we do detect a possible
line of sight structure: it appears only when sources at $z\ga 3$ are used for
reconstructing the mass. We discuss possible origins of the mass-galaxy offsets
in Abell 3827 and Abell 2218: these include pure gravitational effects like
dynamical friction, but also non-standard mechanisms like self-interacting
dark-matter.
