SHARP - IV. An apparent flux ratio anomaly resolved by the edge-on disc
  in B0712+472
  Flux ratio anomalies in quasar lenses can be attributed to dark matter
substructure surrounding the lensing galaxy and, thus, used to constrain the
substructure mass fraction. Previous applications of this approach infer a
substructure abundance that potentially in tension with the predictions of a
$\Lambda$CDM cosmology. However, the assumption that all flux ratio anomalies
are due to substructure is a strong one, and alternative explanations have not
been fully investigated. Here, we use new high-resolution near-IR Keck~II
adaptive optics imaging for the lens system CLASS B0712+472 to perform
pixel-based lens modelling for this system and, in combination with new VLBA
radio observations, show that the inclusion of the disc in the lens model can
explain the flux ratio anomalies without the need for dark matter
substructures. The projected disc mass comprises 16% of the total lensing mass
within the Einstein radius and the total disc mass is $1.79 \times 10^{10}
M_{sun}$. The case of B0712+472 adds to the evidence that not all flux ratio
anomalies are due to dark subhaloes, and highlights the importance of taking
the effects of baryonic structures more fully into account in order to obtain
an accurate measure of the substructure mass fraction.
