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Line-of-sight shear in SLACS strong lenses II: validation tests with an extended sample

Natalie B. Hogg, Daniel Johnson, Anowar J. Shajib, Julien Larena

Abstract

Strong gravitational lensing images are subject to shape distortions due to inhomogeneities along the line of sight. The leading order shape distortion is shear, which, if measurable, will be a complementary cosmological probe to traditional cosmic shear. In Hogg et al. (2025a), we modelled 23 of the SLACS strong lenses, studying the line-of-sight (LOS) shear under a variety of shear and mass model parametrisations. In this work, we model 27 additional lenses, extending our sample of LOS shear constraints to 45 in total. We find a mean shear magnitude of $0.11\pm 0.024$, showing that a significant fraction of the lenses modelled in this work possess LOS shears with unexpectedly large magnitudes, $|γ_{\rm LOS}| > 0.1$, even when an octupolar distortion is included in the lens mass. We further investigate if factors such as lens and source redshift, filter and PSF, or flux and signal-to-noise ratio in the lensed arcs correlate with shear. We find that none of these features play a statistically significant role in the production of unusually large shear magnitudes.

Line-of-sight shear in SLACS strong lenses II: validation tests with an extended sample

Abstract

Strong gravitational lensing images are subject to shape distortions due to inhomogeneities along the line of sight. The leading order shape distortion is shear, which, if measurable, will be a complementary cosmological probe to traditional cosmic shear. In Hogg et al. (2025a), we modelled 23 of the SLACS strong lenses, studying the line-of-sight (LOS) shear under a variety of shear and mass model parametrisations. In this work, we model 27 additional lenses, extending our sample of LOS shear constraints to 45 in total. We find a mean shear magnitude of , showing that a significant fraction of the lenses modelled in this work possess LOS shears with unexpectedly large magnitudes, , even when an octupolar distortion is included in the lens mass. We further investigate if factors such as lens and source redshift, filter and PSF, or flux and signal-to-noise ratio in the lensed arcs correlate with shear. We find that none of these features play a statistically significant role in the production of unusually large shear magnitudes.

Paper Structure

This paper contains 13 sections, 12 equations, 9 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Gravitational lensing formalism
  3. Weak lensing
  4. Shear from strong lenses
  5. The minimal model for line-of-sight shear
  6. Data and methodology
  7. Results and discussion
  8. Large shear magnitudes
  9. What do large and small shear lenses have in common?
  10. Redshift and sky location
  11. Filters and PSF
  12. Flux and signal-to-noise ratio
  13. Conclusions

Figures (9)

  • 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 four lenses fit with the minimal model. The panels show the same information as in \ref{['fig:min1']}.
  • Figure 5: The lenses in our sample for which we consider the modelling using the baseline EPL + minimal LOS shear model to be a failure.
  • ...and 4 more figures