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Cited by 39Galaxies: Formation, Evolution, PhenomenaAstrophysical Phenomena and ObservationsCosmology and Gravitation Theories

Strong gravitational lensing’s ‘external shear’ is not shear

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Etherington, Amy, Nightingale, James W, Massey, Richard, Tam, Sut-Ieng, Cao, XiaoYue, Niemiec, Anna, He, Qiuhan, Robertson, Andrew +15 more·2024·Monthly Notices of the Royal Astronomical Society
DOI 10.1093/mnras/stae1375
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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 along our line of sight. A small amount of external shear could come from these sources, but we show that the vast majority does not. Except in a handful of rare systems, the best-fitting values do not correlate with independent measurements of line-of-sight 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 external shear is aligned with the major or minor axis of 88 per cent of lens galaxies; and the amplitude of the external shear increases if that galaxy is discy. 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.

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  • Etherington et al. demonstrated in galaxy-galaxy lenses that the magnitude and direction of strong-lensing-derived γext shows little correlation with known weak lensing shear, and is instead highly correlated with the elliptical orientation of the lens itself.

    “While the motivation of this approach is to generically capture and account for any such perturbations, Etherington et al. [82] has demonstrated in galaxy-galaxy lenses that the magnitude and direction of SL-derived γ ext shows little correlation with known weak lensing shear, and is instead highly correlated with the elliptical orientation of the lens itself.”

    From Constraint on dark matter self-interaction from combined strong lensing and stellar kinematics in MACS J0138-2155 (2026) · cites this paper as Etherington et al. [82]

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Cited by

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Authors
Etherington, A., Nightingale, J. W., Massey, R., Tam, S.-I., Cao, X., Niemiec, A., He, Q., Robertson, A., Li, R., Amvrosiadis, A., Cole, S., Diego, J. M., Frenk, C. S., Frye, B. L., Harvey, D., Jauzac, M., Koekemoer, A. M., Lagattuta, D. J., Lange, S., Limousin, M., Mahler, G., Sirks, E., & Steinhardt, C. L.
Affiliations
  • Durham University (GB)
  • Institute of Astronomy and Astrophysics, Academia Sinica (TW)
  • Chinese Academy of Sciences (CN)
  • California Institute of Technology (US)
  • Consejo Superior de Investigaciones Científicas (ES)
  • University of Arizona (US)
  • École Polytechnique Fédérale de Lausanne (CH)
  • Centre National de la Recherche Scientifique (FR)
  • The University of Sydney (AU)
  • University of Copenhagen (DK)
Corresponding author
Nightingale, James W
ORCID
Venue
Monthly Notices of the Royal Astronomical Society
Publisher
Oxford University Press (OUP)
Published
2024-06-03
Volume
531
Issue
3
Pages
3684-3697
ISSN
  • 0035-8711 (ISSN-L)
  • 1365-2966
DOI
10.1093/mnras/stae1375
OpenAlex
W4399322435
Cited by
39
References
1
Citations by year
2026: 8 · 2025: 19 · 2024: 12
FWCI
6.72
Type
journal article
Language
en
Open access
Licence
Locations
  • Monthly Notices of the Royal Astronomical Society (publishedVersion, cc-by)
  • Durham Research Online (Durham University) (publishedVersion, other-oa)
  • Research at the University of Copenhagen (University of Copenhagen) (publishedVersion, cc-by)
  • HAL AMU (submittedVersion, cc-by)
  • ORBi (University of Liège) (submittedVersion, other-oa)
  • DIGITAL.CSIC (Spanish National Research Council (CSIC)) (submittedVersion, cc-by)
  • Infoscience (Ecole Polytechnique Fédérale de Lausanne) (submittedVersion, cc-by-nc-nd)
Funders
  • STFC (ST/R504725/1, ST/T506047/1, ST/T002565/1, ST/W002612/1, ST/P002307/1, ST/R002452/1, ST/H008519/1, ST/K00087X/1, ST/K003267/1)
  • National Natural Science Foundation of China (11988101, 11773032, 12022306, CMS-CSST-2021-B01, CMS-CSST-2021-A01)
  • K. C. Wong Education Foundation
  • ERC (GA 786910)
  • UKRI (MR/S017216/1)
  • BIS (ST/K00042X/1)
  • Durham University
Topics
  • Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Galaxies: Formation, Evolution, Phenomena
  • Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Astrophysical Phenomena and Observations
  • Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Cosmology and Gravitation Theories
Keywords
strong gravitational lensing, external shear, galaxy-scale lenses, lens mass models, weak gravitational lensing, disk galaxies, model bias, dark matter

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