The Effect of Environment on Shear in Strong Gravitational Lenses

TL;DR

This study investigates how the environment around strong gravitational lenses influences shear, revealing that line-of-sight galaxies significantly affect lensing measurements and highlighting the need for detailed environmental data in future surveys.

Contribution

It provides the first detailed independent check of shear from lens environment measurements and explores the impact of local and line-of-sight structures on lens modeling.

Findings

Environment induces an average shear of 0.08, affecting lens observables.

Line-of-sight galaxies contribute non-negligible shear, averaging 0.05.

Discrepancies between environmental shear and lens model shear suggest model assumptions need reevaluation.

Abstract

Using new photometric and spectroscopic data in the fields of nine strong gravitational lenses that lie in galaxy groups, we analyze the effects of both the local group environment and line-of-sight galaxies on the lens potential. We use Monte Carlo simulations to derive the shear directly from measurements of the complex lens environment, providing the first detailed independent check of the shear obtained from lens modeling. We account for possible tidal stripping of the group galaxies by varying the fraction of total mass apportioned between the group dark matter halo and individual group galaxies. The environment produces an average shear of gamma = 0.08 (ranging from 0.02 to 0.17), significant enough to affect quantities derived from lens observables. However, the direction and magnitude of the shears do not match those obtained from lens modeling in three of the six 4-image systems in our sample (B1422, RXJ1131, and WFI2033). The source of this disagreement is not clear, implying that the assumptions inherent in both the environment and lens model approaches must be reconsidered. If only the local group environment of the lens is included, the average shear is gamma = 0.05 (ranging from 0.01 to 0.14), indicating that line-of-sight contributions to the lens potential are not negligible. We isolate the effects of various theoretical and observational uncertainties on our results. Of those uncertainties, the scatter in the Faber-Jackson relation and error in the group centroid position dominate. Future surveys of lens environments should prioritize spectroscopic sampling of both the local lens environment and objects along the line of sight, particularly those bright (I < 21.5) galaxies projected within 5' of the lens.