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Galaxy–galaxy strong lens perturbations: line-of-sight haloes versus lens subhaloes

He, Qiuhan; Li, Ran; Frenk, Carlos S; Nightingale, James; Cole, Shaun; Amorisco, Nicola C; Massey, Richard; Robertson, Andrew; Etherington, Amy; Amvrosiadis, Aristeidis; Cao, Xiaoyue

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Dr Qiuhan He
Post Doctoral Research Associate

Ran Li

Nicola C Amorisco

Andrew Robertson

Xiaoyue Cao


We rederive the number density of intervening line-of-sight haloes relative to lens subhaloes in galaxy-galaxy strong lensing observations, where these perturbers can generate detectable image fluctuations. Previous studies have calculated the detection limit of a line-of-sight small-mass dark halo by comparing the lensing deflection angles it would cause, to those caused by a subhalo within the lens. However, this overly simplifies the difference in observational consequences between a subhalo and a line-of-sight halo. Furthermore, it does not take into account degeneracies between an extra subhalo and the uncertain properties of the main lens. More in keeping with analyses of real-world observations, we regard a line-of-sight halo as detectable only if adding it to a smooth model generates a statistically significant improvement in the reconstructed image. We find that the number density of detectable line-of-sight perturbers has been overestimated by as much as a factor of two in the previous literature. For typical lensing geometries and configurations, very deep imaging is sensitive to twice as many line-of-sight perturbers as subhaloes, but moderate depth imaging is sensitive to only slightly more line-of-sight perturbers than subhaloes.


He, Q., Li, R., Frenk, C. S., Nightingale, J., Cole, S., Amorisco, N. C., …Cao, X. (2022). Galaxy–galaxy strong lens perturbations: line-of-sight haloes versus lens subhaloes. Monthly Notices of the Royal Astronomical Society, 512(4), 5862-5873.

Journal Article Type Article
Acceptance Date Mar 11, 2022
Online Publication Date Mar 21, 2022
Publication Date 2022-06
Deposit Date Jul 12, 2022
Publicly Available Date Jul 12, 2022
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Electronic ISSN 1365-2966
Publisher Royal Astronomical Society
Peer Reviewed Peer Reviewed
Volume 512
Issue 4
Pages 5862-5873


Published Journal Article (1.8 Mb)

Copyright Statement
This article has been accepted for publication in Monthly notices of the Royal Astronomical Society. ©: 2022 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

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