Line-of-sight effects in strong gravitational lensing

Abstract

While most strong-gravitational-lensing systems may be roughly modelled by a single massive object between the source and the observer, in the details all the structures near the light path contribute to the observed images. These additional contributions, known as line-of-sight effects, are non-negligible in practice. This article proposes a new theoretical framework to model the line-of-sight effects, together with very promising applications at the interface of weak and strong lensing. Our approach relies on the dominant-lens approximation, where one deflector is treated as the main lens while the others are treated as perturbations. The resulting framework is technically simpler to handle than the multi-plane lensing formalism, while allowing one to consistently model any kind of perturbation. In particular, it is not limited to the usual external-convergence and external-shear parameterisation. As a first application, we identify a specific notion of line-of-sight shear that is not degenerate with the ellipticity of the main lens, and which could thus be extracted from strong-lensing images. This result supports and improves the recent proposal that Einstein rings might be powerful probes of cosmic shear. As a second application, we investigate the distortions of strong-lensing critical curves under line-of-sight effects, and more particularly their correlations across the sky. We find that such correlations may be used to probe, not only the large-scale structure of the Universe, but also the dark-matter halo profiles of strong lenses. This last possibility would be a key asset to improve the accuracy of the measurement of the Hubble-Lemaître constant via time-delay cosmography.

ALMA Measurement of 10 kpc-scale Lensing Power Spectra towards the Lensed Quasar MG J0414+0534

“…A subsequent analysis obtained a similar result (Despali et al 2018). Frameworks for modelling 'LOS lensing' have been explored in the literature (Erdl & Schneider 1993;Bar-Kana 1996;McCully et al 2014McCully et al , 2017Birrer et al 2017;Fleury et al 2021). LOS lensing has been used to constrain warm, mixed, and other dark matter models (Inoue et al 2015;Kamada et al 2016Kamada et al , 2017.…”

“…We did not include flexion (third derivative of potential) terms in describing potential fluctuations of massive objects located far from the lens(Okura et al 2007;McCully et al 2017;Fleury et al 2021) as they can be considered as low frequency Fourier modes. Similarly, we did not consider the third or fourth order multipoles m = 3 and m = 4 explicitly.…”

Accurate modelling of extragalactic microlensing by compact objects

“…The lens equation is the relation between θ and β. For a dominant point lens with tidal perturbation along the line of sight, it takes the form [66][67][68][69][70][71][72][73]…”

“…(3.9), s ab , is the microshear produced by compact objects in the vicinity of the line of sight, except the dominant lens. If the region between (a) and (b) contains N point-like lenses labelled with , then the microshear reads [73]…”

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

“…In the lower panel we plot the change in the total deflection angles when a 𝑚 200 = 10 8.67 M NFW halo at 𝑧 = 0.2 perturbs the lensing due to the main lens (subtracting the total deflection angle with the deflection caused only by the main lens). Because of multi-plane lensing effects (Schneider et al 1992;Fleury et al 2021) the change in the deflection angles are no longer isotropic about the perturber centre (Gilman et al 2019;He et al 2020). Tracing from the observer backwards, the deflection of light rays by the perturber alters where those rays intersect the main lens plane, which in turn alters the deflection angles those rays receives from the main lens.…”