The Effect of Environment on Shear in Strong Gravitational Lenses

Publication Title

The Astrophysical Journal

Document Type



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 structures on the lens potential. We derive the shear directly from measurements of the lens environment, independent of the shear obtained from lens modeling. We account for possible tidal stripping of the group galaxies by allowing the fraction of total mass apportioned to the group dark matter halo and the individual group galaxies to vary freely. The environment produces an average shear of γ = 0.14 ± 0.04, significant enough to affect quantities derived from lens observables. However, the direction and magnitude of the shears derived from the environment does not match those obtained via lens modeling in three of the six 4-image systems where we have calculated model shears. 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 γ = 0.08 ± 0.03, 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, scatter in the Faber-Jackson relation dominates, boosting the scatter in the shear components γc and γs by as much as 0.04. Error in the group centroid position has a large effect on lenses near the centers of their respective groups, resulting in an offset in γc or γs of as much as 0.06. Future surveys of lens environments should prioritize spectroscopic sampling of both the local lens environment and objects along the line of sight, particularly bright (I 〈 21.5) objects projected within 2' of the lens.


Physics and Astronomy










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