2025MNRAS.539..704L

We analyse two galaxy-scale strong gravitational lenses, SPT0418-47 and SPT2147-50, using JWST NIRCam imaging across multiple filters. To account for angular complexity in the lens mass distribution, we introduce multipole perturbations with orders $m=1, 3, 4$. Our results show strong evidence for angular mass complexity in SPT2147, with multipole strengths of 0.3-1.7 per cent for $m=3, 4$ and 2.4-9.5 per cent for $m=1$, while SPT0418 shows no such preference. We also test lens models that include a dark matter substructure, finding a strong preference for a substructure in SPT2147-50 with a Bayes factor of $\Delta {\rm log}( { { {\varepsilon }}})$$\sim 60$ when multipoles are not included. Including multipoles reduces the Bayes factor to $\Delta {\rm log}( { { {\varepsilon }}})$$\sim 11$, still corresponding to a $5\sigma$ detection of a subhalo with an NFW (Navarro-Frenk-White) mass of $\log _{10}(M_{200}/{\rm M}_{\odot }) = 10.87^{+0.53}_{-0.71}$. While SPT2147-50 may represent the fourth detection of a dark matter substructure in a strong lens, further analysis is needed to confirm that the signal is not due to systematics associated with the lens mass model.