TY - JOUR
T1 - Strong gravitational lensing's 'external shear' is not shear
AU - Etherington, Amy
AU - Nightingale, James W.
AU - Massey, Richard
AU - Tam, Sut Ieng
AU - Cao, Xiao Yue
AU - Niemiec, Anna
AU - He, Qiuhan
AU - Robertson, Andrew
AU - Li, Ran
AU - Amvrosiadis, Aristeidis
AU - Cole, Shaun
AU - Diego, Jose M.
AU - Frenk, Carlos S.
AU - Frye, Brenda L.
AU - Harvey, David
AU - Jauzac, Mathilde
AU - Koekemoer, Anton M.
AU - Lagattuta, David J.
AU - Lange, Samuel
AU - Limousin, Marceau
AU - Mahler, Guillaume
AU - Sirks, Ellen
AU - Steinhardt, Charles L.
N1 - Publisher Copyright:
© 2024 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society.
PY - 2024/7/1
Y1 - 2024/7/1
N2 - 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.
AB - 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.
KW - galaxies: structure
KW - gravitational lensing: strong
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U2 - 10.1093/mnras/stae1375
DO - 10.1093/mnras/stae1375
M3 - Article
AN - SCOPUS:85196072581
SN - 0035-8711
VL - 531
SP - 3684
EP - 3697
JO - Monthly Notices of the Royal Astronomical Society
JF - Monthly Notices of the Royal Astronomical Society
IS - 3
ER -