SHARP - II. Mass structure in strong lenses is not necessarily dark
  matter substructure: A flux ratio anomaly from an edge-on disc
  Gravitational lens flux-ratio anomalies provide a powerful technique for
measuring dark matter substructure in distant galaxies. However, before using
these flux-ratio anomalies to test galaxy formation models, it is imperative to
ascertain that the given anomalies are indeed due to the presence of dark
matter substructure and not due to some other component of the lensing galaxy
halo or to propagation effects. Here we present the case of CLASS~B1555+375,
which has a strong radio-wavelength flux-ratio anomaly. Our high-resolution
near-infrared Keck~II adaptive optics imaging and archival Hubble Space
Telescope data reveal the lensing galaxy in this system to have a clear edge-on
disc component that crosses directly over the pair of images that exhibit the
flux-ratio anomaly. We find that simple models that include the disc can
reproduce the cm-wavelength flux-ratio anomaly without requiring additional
dark matter substructure. Although further studies are required, our results
suggest the assumption that all flux-ratio anomalies are due to a population of
dark matter sub-haloes may be incorrect, and analyses that do not account for
the full complexity of the lens macro-model may overestimate the substructure
mass fraction in massive lensing galaxies.
