Abstract
Gravitational lens models composed of a single smooth mass profile leave significant position and flux ratio or magnification anomalies commonly attributed to real galaxies having line-of-sight effects and dark matter substructure unaccounted for by smooth profiles. Lens models have therefore evolved to invoke an external shear in addition to the smooth profile to account for said line-of-sight effects. Although this external shear completely resolves positional anomalies in nearly all observed systems, the shear strengths used in such lens models are too strong and completely unphysical. In this work, we show how and why the external shear can always resolve positional anomalies to arbitrary precision by using mock lens models generated from the IllustrisTNG Simulations. The lack of any line-of-sight effects in these simulated lenses allows us to perfectly capture how the external shear manipulates the smooth model to force the lensed images to fall into a curling or twisting configuration. We furthermore show that the best-fit external shear strengths in systems with no line-of-sight effects are comparable to shear strengths we obtain in real lenses. This implies that the traditional lens modelling approach of blindly adding external shear is misleading in that it hides the anomalies present in real lenses and can result in misleading galaxy mass distributions. These results indicate that position and magnification anomalies still exist in lens models which changes the nature of dark matter substructure commonly used to resolve the remaining lensing anomalies.
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