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

TL;DR

This study extends the analysis of line-of-sight shear in strong gravitational lenses, finding unexpectedly large shear magnitudes that are not explained by common observational factors, thus providing new insights into lens modeling.

Contribution

The paper expands the sample of LOS shear measurements to 45 lenses and investigates factors influencing shear magnitudes, revealing unexpectedly large shears not correlated with typical observational parameters.

Findings

Mean shear magnitude of 0.11 with significant large shear cases

Large shear magnitudes (>0.1) are common even with complex lens models

No significant correlation between shear and observational features

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, $|\gamma_{\rm LOS}| > 0.1$, 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.