Galaxy mass modelling from multiwavelength JWST strong lens analysis: dark matter substructure, angular mass complexity, or both?

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 Olog(e)'60 when multipoles are not included. Including multipoles reduces the Bayes factor to Olog(e)'11, still corresponding to a 5cr detection of a subhalo with an NFW (Navarro-Frenk-White) mass of log10(M-200/M-(R)) = 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.