# Strong gravitational lensing's `external shear' is not shear

**Authors:** Amy Etherington, James W. Nightingale, Richard Massey, Sut-Ieng Tam, XiaoYue Cao, Anna Niemiec, Qiuhan He, Andrew Robertson, Ran Li, Aristeidis Amvrosiadis, Shaun Cole, Jose M. Diego, Carlos S. Frenk, Brenda L. Frye, David Harvey, Mathilde Jauzac, Anton M. Koekemoer, David J. Lagattuta, Marceau Limousin, Guillaume Mahler  
**arXiv:** [2301.05244](https://arxiv.org/abs/2301.05244)  
**Date:** 2023-01-12  
**Summary page:** https://www.summarizepaper.com/en/paper/2301.05244v1/

## Key points

- Distribution of mass in galaxy-scale strong gravitational lenses is often modelled as an elliptical power law plus 'external shear'
- External shear accounts for neighbouring galaxies and cosmic shear
- Recent study by Amy Etherington et al. shows that external shear is not physically meaningful but rather a fudge to compensate for lack of model complexity
- Best-fit values of external shear do not correlate with independent measurements of shear from weak lensing in 45 Hubble Space Telescope images or in 50 mock images of lenses with complex distributions of mass, except in rare systems
- Best-fit shear is aligned with the major or minor axis of 88% of lens galaxies, and the amplitude of the external shear increases if that galaxy is disky
- This finding has significant implications since it biases other model parameters that are interpreted as physically meaningful in several science analyses such as measuring galaxy evolution, dark matter physics, or cosmological parameters
- Future studies should employ more flexible mass models instead of relying on the current power law plus external shear model to ensure accurate results from these science analyses.

## Summary

The distribution of mass in galaxy-scale strong gravitational lenses is often modelled as an elliptical power law plus `external shear', which accounts for neighbouring galaxies and cosmic shear. However, a recent study by Amy Etherington et al., published in the Monthly Notices of the Royal Astronomical Society (MNRAS), shows that this external shear is not physically meaningful but rather a fudge to compensate for lack of model complexity. The paper includes 13 pages and 11 figures providing evidence for their claims about external shear's lack of physical meaning. The best-fit values of external shear do not correlate with independent measurements of shear from weak lensing in 45 Hubble Space Telescope images or in 50 mock images of lenses with complex distributions of mass, except in rare systems. Instead, the best-fit shear is aligned with the major or minor axis of 88% of lens galaxies, and the amplitude of the external shear increases if that galaxy is disky. This finding has significant implications since it biases other model parameters that are interpreted as physically meaningful in several science analyses such as measuring galaxy evolution, dark matter physics, or cosmological parameters. Therefore, future studies should employ more flexible mass models instead of relying on the current power law plus external shear model to ensure accurate results from these science analyses.