Line-of-sight effects in strong lensing: Putting theory into practice

TL;DR

This paper introduces a method to accurately separate line-of-sight weak lensing effects from strong lensing signals in galaxy systems, improving cosmic shear measurements and halo mass estimates.

Contribution

The authors develop a simple, image-based technique to disentangle LOS effects from strong lensing, validated with mock data and applied to real systems, enhancing lensing analysis accuracy.

Findings

Achieved shear measurement precision of ±0.003 with mock data.

Joint analysis improves convergence estimates by a factor of two.

Tighter halo mass constraints from combined strong lens and LOS data.

Abstract

We present a simple method to accurately infer line of sight (LOS) integrated lensing effects for galaxy scale strong lens systems through image reconstruction. Our approach enables us to separate weak lensing LOS effects from the main strong lens deflector. We test our method using mock data and show that strong lens systems can be accurate probes of cosmic shear with a precision on the shear terms of $\pm 0.003$ (statistical error) for an HST-like dataset. We apply our formalism to reconstruct the lens COSMOS 0038+4133 and its LOS. In addition, we estimate the LOS properties with a halo-rendering estimate based on the COSMOS field galaxies and a galaxy-halo connection. The two approaches are independent and complementary in their information content. We find that when estimating the convergence at the strong lens system, performing a joint analysis improves the measure by a factor of two compared to a halo model only analysis. Furthermore the constraints of the strong lens reconstruction lead to tighter constraints on the halo masses of the LOS galaxies. Joint constraints of multiple strong lens systems may add valuable information to the galaxy-halo connection and may allow independent weak lensing shear measurement calibrations.