Systematic or Signal? How dark matter misalignments can bias strong
  lensing models of galaxy clusters
  We explore how assuming that mass traces light in strong gravitational
lensing models can lead to systematic errors in the predicted position of
multiple images. Using a model based on the galaxy cluster MACSJ0416 (z =
0.397) from the Hubble Frontier Fields, we split each galactic halo into a
baryonic and dark matter component. We then shift the dark matter halo such
that it no longer aligns with the baryonic halo and investigate how this
affects the resulting position of multiple images. We find for physically
motivated misalignments in dark halo position, ellipticity, position angle and
density profile, that multiple images can move on average by more than 0.2"
with individual images moving greater than 1". We finally estimate the full
error induced by assuming that light traces mass and find that this assumption
leads to an expected RMS error of 0.5", almost the entire error budget observed
in the Frontier Fields. Given the large potential contribution from the
assumption that light traces mass to the error budget in mass reconstructions,
we predict that it should be possible to make a first significant detection and
characterisation of dark halo misalignments in the Hubble Frontier Fields with
strong lensing. Finally, we find that it may be possible to detect ~1kpc
offsets between dark matter and baryons, the smoking gun for self-interacting
dark matter, should the correct alignment of multiple images be observed.
