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 lensed image that is bent by an edge-on lenticular galaxy, in the Hubble Frontier Fields (HFF) data of MACSJ0416.1-20403. The fortuitous combination of a long arc (zs ≈ 1 ± 0.2) intersecting an edge-on galaxy from the cluster (z = 0.4) provides an opportunity to constrain its dark matter (DM) halo and its orientation. We model the stellar lensing contribution and we add to this a standard parametrized dark halo component. Irrespective of the detailed choice of parameters, we obtain a combined total mass of ≈3 × 1011M⊙. Depending on the dark halo parameters, the stellar contribution to this is limited to the range 5-15 × 1010M⊙, or 20-50 per cent of the total mass, in good agreement with the independent (photmetric) stellar mass of 5 × 1010M⊙ (Chabrier IMF), or 8 × 1010M⊙ (Salpeter IMF). The major axis of theDMhalo is constrained to be nearly orthogonal to the plane of the galaxy, and with an ellipticity e ≈ 0.15 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 that do not require DM are challenged by this finding since generically these must be tied to the baryonic component which is highly disfavoured by our results. Other such fortuitously useful lenses can be examined this way as they become uncovered with more HFF data to help provide a more statistical distribution of galaxy halo properties. © 2015 The Authors.