Line-of-sight shear in SLACS strong lenses

Abstract: Inhomogeneities along the line of sight in strong gravitational lensing distort the images produced, in an effect called shear. If measurable, this shear may provide independent constraints on cosmological parameters, complementary to traditional cosmic shear. We model 50 strong gravitational lenses from the Sloan Lens ACS (SLACS) catalogue with the aim of measuring the line-of-sight (LOS) shear for the first time. We use the `minimal model' for the LOS shear, which has been shown to be theoretically safe from degeneracies with lens model parameters, a finding which has been confirmed using mock data. We use the dolphin automated modelling pipeline, which uses the lenstronomy software as a modelling engine, to model our selected lenses. We model the main deflector with an elliptical power law profile, the lens light with elliptical S\'ersic profiles and the source with a basis set of shapelets and an elliptical S\'ersic profile. We successfully obtain a line-of-sight shear measurement from 18 of the 50 lenses. We find that these LOS shear measurements are consistent with external shears measured in recent works using a simpler shear model, which are larger than those expected from weak lensing. Neglecting the post-Born correction to the potential of the main deflector due to foreground shear leads to a propagation of degeneracies to the LOS shear measurement, and the same effect is seen if a prior is used to connect the lens mass and light ellipticities. The inclusion of an octupole moment in the lens mass profile does not lead to shear measurements that are in better agreement with the expectations from weak lensing.