A lensed radio jet at milliarcsecond resolution I: Bayesian comparison of parametric lens models

We investigate the mass structure of a strong gravitational lens galaxy at $z=0.350$, taking advantage milli-arcsecond (mas) angular resolution very long baseline interferometric (VLBI) observations. In first analysis its kind this resolution, we jointly infer model parameters and pixellated radio source surface brightness. consider several models increasing complexity, starting from an elliptical power-law density profile. extend to include multipole structures, separate stellar component, additional nearby field galaxies, and/or generic external potential. compare these using their relative Bayesian log-evidence (Bayes factor). find evidence for in lens; our best is comprised profile plus perturbations potential, with Bayes factor $+14984$ model. It noteworthy that distribution remarkably good fit on own, complexity correcting deflection angles only $\sim5$ mas level. also effects added time-delay cosmography flux-ratio analyses. overly simplistic ellipsoid can bias measurement $H_0$ by $\sim3$ per cent mimic flux ratio anomalies $\sim8$ cent. Our results demonstrate power high-resolution VLBI observations provide constraints inner profiles galaxies.