Strong-lensing and kinematic analysis of CASSOWARY 31: can strong lensing constrain the masses of multi-plane lenses?

TL;DR

This study investigates how strong lensing can be used to measure the mass of secondary, multi-plane lenses, highlighting the importance of accounting for line-of-sight structures to reduce systematic uncertainties.

Contribution

It demonstrates the impact of line-of-sight structures on mass estimates in multi-plane strong lensing models and emphasizes the need for detailed line-of-sight analysis.

Findings

Strong lensing predictions tend to overestimate secondary lens mass.

Including additional lens planes improves agreement with kinematic measurements.

Systematic uncertainties arise from degeneracies between lens planes and complex line-of-sight structures.

Abstract

We present a mass measurement for the secondary lens along the line of sight (LoS) in the multi-plane strong lens modeling of the group-scale lens CASSOWARY 31 (CSWA 31). The secondary lens at redshift $z = 1.49$ is a spiral galaxy well aligned along the LoS with the main lens at $z = 0.683$. Using the MUSE integral-field spectroscopy of this spiral galaxy, we measure its rotation velocities and determine the mass from the gas kinematics. We compare the mass estimation of the secondary lens from the lensing models to the mass measurement from kinematics, finding that the predictions from strong lensing tend to be higher. By introducing an additional lens plane at $z = 1.36$ for an overdensity known to be present, we find a mass of $\simeq 10^{10}$ M$_\odot$ enclosed within 3.3 kpc from the centroid of the spiral galaxy, approaching the estimate from kinematics. This shows that secondary-lens mass measurements from multiple-plane modeling are affected by systematic uncertainties from the degeneracies between lens planes and the complex LoS structure. Conducting a detailed analysis of the LoS structures is therefore essential to improve the mass measurement of the secondary lens.