Line-of-sight shear in SLACS strong lenses I: shear and mass model parametrisations

TL;DR

This work assesses the feasibility of measuring line-of-sight (LOS) shear in strong gravitational lenses as a cosmological probe. Using the minimal LOS shear model to prevent degeneracies with the main lens mass, the authors model 23 SLACS lenses with a pipeline built on lenstronomy via dolphin, extracting $\gamma_{\rm LOS}$. They find a mean LOS-shear magnitude $\lvert \gamma_{\rm LOS} \rvert = 0.056 \pm 0.016$, consistent with external-shear expectations and simulations but generally larger than weak-lensing predictions; incorporating octupolar mass distortions does not universally reduce LOS shear, and neglecting post-Born corrections introduces only modest biases. The results emphasize the necessity of robust lens-mass modelling for cosmological inferences from strong-lensing shear, while highlighting ongoing challenges in capturing the full mass complexity of lens galaxies and data limitations that currently constrain large-sample analyses.

Abstract

Inhomogeneities along the line of sight in strong gravitational lensing distort the images produced, in an effect called shear. If measurable, this shear may provide independent constraints on cosmological parameters, complementary to traditional cosmic shear. We model 23 strong gravitational lenses from the Sloan Lens ACS (SLACS) catalogue with the aim of measuring the line-of-sight (LOS) shear for the first time. We use the 'minimal model' for the LOS shear, which has been shown to be theoretically safe from degeneracies with lens model parameters, a finding which has been confirmed using mock data. We use the dolphin automated modelling pipeline, which uses the lenstronomy software as a modelling engine, to model our selected lenses. Across the 23 lenses, we measure the LOS shear with a mean magnitude of $0.056 \pm 0.013$. Neglecting the post-Born correction to the potential of the main deflector due to foreground shear leads to a propagation of degeneracies in the LOS shear measurement with other lens model parameters, and the inclusion of an octupole moment in the lens mass profile does not lead to shear measurements that are in better agreement with the expectations from weak lensing.

Figures (10)

• Figure 1: The first six lenses fit with the minimal model. From left to right, the panels show the single-band image data for each lens, our reconstruction of the image along with the reduced $\chi^2$ of the model, the residual difference between the image and the reconstruction, the reconstructed source and the one dimensional marginalised posterior distribution of the LOS shear magnitude, $|\gamma_{\rm LOS}|$. The shaded area is the $1 \sigma$ confidence interval.
• Figure 2: The next six lenses fit with the minimal model. The panels show the same information as in \ref{['fig:min1']}.
• Figure 3: The next six lenses fit with the minimal model. The panels show the same information as in \ref{['fig:min1']}.
• Figure 4: The final five lenses fit successfully with the minimal model. The panels show the same information as in \ref{['fig:min1']}.
• Figure 5: This figure shows the histograms of the $|\gamma_{\rm LOS}|$ values measured in this work (solid blue) and the external shear values measured in Shajib2021 (unfilled black).