  The flux ratios of strongly lensed quasars have previously been used to infer
the properties of dark matter. In these analyses it is crucial to separate the
effect of the main lensing galaxy and the low-mass dark matter halo population.
In this work, we investigate flux-ratio perturbations resulting from general
third- and fourth-order multipole perturbations to the main lensing galaxy's
mass profile. We simulate four lens systems, each with a different lensing
configuration, without multipoles. The simulated flux ratios are perturbed by
10-40 per cent by a population of low-mass haloes consistent with CDM and, in
one case, also a satellite galaxy. This level of perturbation is comparable to
the magnitude of flux-ratio anomalies in real data that has been previously
analyzed. We then attempt to fit the simulated systems using multipoles instead
of low-mass haloes. We find that multipoles with amplitudes of 0.01 or less can
produce flux-ratio perturbations in excess of 40 per cent. In all cases, third-
or fourth-order multipoles can individually reduce the magnitude of, if not
eliminate, flux-ratio anomalies. When both multipole orders are jointly
included, all simulated flux ratios can be fit to within the observational
uncertainty. Our results indicate that low-mass haloes and multipoles are
highly degenerate when modelling quadruply-imaged quasars based just on image
positions and flux ratios. In the presence of this degeneracy, flux-ratio
anomalies in lensed quasars alone cannot be used to place strong constraints on
the properties of dark matter without additional information that can inform
our priors.
