A free-form lensing model of A370 revealing stellar mass dominated BCGs,
  in Hubble Frontier Fields images
  We derive a free-form mass distribution for the unrelaxed cluster A370
(z=0.375), using the latest Hubble Frontier Fields images and GLASS
spectroscopy. Starting from a reliable set of 10 multiply lensed systems we
produce a free-form lens model that identifies ~ 80 multiple-images. Good
consistency is found between models using independent subsamples of these
lensed systems, with detailed agreement for the well resolved arcs. The mass
distribution has two very similar concentrations centred on the two prominent
Brightest Cluster Galaxies (or BCGs), with mass profiles that are accurately
constrained by a uniquely useful system of long radially lensed images centred
on both BCGs. We show that the lensing mass profiles of these BCGs are mainly
accounted for by their stellar mass profiles, with a modest contribution from
dark matter within r<100 kpc of each BCG. This conclusion may favour a cooled
cluster gas origin for BCGs, rather than via mergers of normal galaxies for
which dark matter should dominate over stars. Growth via merging between BCGs
is, however, consistent with this finding, so that stars still dominate over
dark matter .
