The Orthogonally Aligned Dark Halo of an Edge-on Lensing Galaxy in the
  Hubble Frontier Fields: A Challenge for Modified Gravity
  We examine a well resolved 8 arcsec lensed image that is symmetrically bent
in the middle by an edge-on lenticular galaxy, in the Hubble Frontier Field
(HFF) data of MACSJ0416.1-20403. The lengthy image is generated primarily by
the large tangential shear of the cluster with a local secondary deflection by
the member galaxy out to a limiting radius of about 18 kpc. The lensing
lenticular galaxy is also well resolved and evidently lies nearly edge-on in
projection. This fortuitous combination of a long arc intersecting an edge on
galaxy provides us with an opportunity to place relatively strong constraints
on the lensing effect of this galaxy. We can model the stellar lensing
contribution using the observed pixels belonging to the galaxy, in 2D, and we
add to this a standard parameterised dark halo component. Irrespective of the
detailed choice of parameters we obtain a combined total mass of about 3E11
Msun. Depending on the dark halo parameters, the stellar contribution to this
is limited to the range 5-15E10 Msun or 20-50 percent of the total mass, in
good agreement with the independent stellar mass computed from the photometry
of 5E10 Msun for a Chabrier IMF, or 8E10Msun for a Salpeter IMF. The major axis
of the DM halo is constrained to be nearly orthogonal to the plane of the
galaxy, within a range of about 15 degrees, and with an ellipticity e=015
corresponding to an axis ratio a/c=0.54. We show that these conclusions are
very weakly dependent on the model of the cluster, or the additional influence
of neighbouring galaxies or the properties of the lensed source. Alternative
theories of gravity where the radial dependence is modified to avoid the need
for DM are challenged by this finding since generically these must be tied to
the baryonic component which here is a stellar disk oriented nearly
orthogonally to the lensed image deflection.
