Abstract:
The large-scale mass distributions of galaxy-scale strong lenses have long been assumed to be well described by a singular ellipsoidal
power-law density profile with external shear. However, the inflexibility of this model could lead to systematic errors in astrophysical
parameters inferred with gravitational lensing observables. Here, we present observations with the Atacama Large (sub-)Millimetre
Array (ALMA) of three strongly lensed dusty star-forming galaxies at '30 mas angular resolution and investigate the sensitivity of
these data to angular structure in the lensing galaxies. We jointly infer the lensing mass distribution and the full surface brightness of
the lensed sources with multipole expansions of the power-law density profile up to the fourth order using a technique developed for
interferometric data. All three datasets strongly favour third and fourth-order multipole amplitudes of ≈1 percent of the convergence.
While the infrared stellar isophotes and isodensity shapes agree for one lens system, for the other two the isophotes disagree to varying
extents, suggesting contributions to the angular structure from dark matter intrinsic or extrinsic to the lensing galaxy.
