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

分类： 天文学 >> 天文学 提交时间： 2023-02-19

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

摘要： 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.

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