Strong gravitational lensing's 'external shear' is not shear
Abstract
The distribution of mass in galaxy-scale strong gravitational lenses is often modelled as an elliptical power law plus 'external shear', which notionally accounts for neighbouring galaxies and cosmic shear. We show that it does not. Except in a handful of rare systems, 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. 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. We conclude that 'external shear' attached to a power law model is not physically meaningful, but a fudge to compensate for lack of model complexity. Since it biases other model parameters that are interpreted as physically meaningful in several science analyses (e.g. measuring galaxy evolution, dark matter physics or cosmological parameters), we recommend that future studies of galaxy-scale strong lensing should employ more flexible mass models.
Cite
@article{arxiv.2301.05244,
title = {Strong gravitational lensing's 'external shear' is not shear},
author = {Amy Etherington and James W. Nightingale and Richard Massey and Sut-Ieng Tam and XiaoYue Cao and Anna Niemiec and Qiuhan He and Andrew Robertson and Ran Li and Aristeidis Amvrosiadis and Shaun Cole and Jose M. Diego and Carlos S. Frenk and Brenda L. Frye and David Harvey and Mathilde Jauzac and Anton M. Koekemoer and David J. Lagattuta and Marceau Limousin and Guillaume Mahler and Ellen Sirks and Charles L. Steinhardt},
journal= {arXiv preprint arXiv:2301.05244},
year = {2023}
}
Comments
13 pages, 11 figures, submitted to MNRAS and Review Response Submitted