A flux-ratio anomaly in the CO spectral line emission from
  gravitationally-lensed quasar MG J0414+0534
  We present an analysis of archival observations with the Atacama Large
(sub-)Millimetre Array (ALMA) of the gravitationally lensed quasar MG
J0414+0534, which show four compact images of the quasar and an Einstein ring
from the dust associated with the quasar host galaxy. We confirm that the
flux-ratio anomalies observed in the mid-infrared and radio persists into the
sub-mm for the continuum images of the quasar. We report the detection of CO
(11-10) spectral line emission, which traces a region of compact gas around the
quasar nucleus. This line emission also shows evidence of a flux-ratio anomaly
between the merging lensed images that is consistent with those observed at
other wavelengths, suggesting high-excitation CO can also provide a useful
probe of substructures that is unaffected by microlensing or dust extinction.
However, we do not detect the candidate dusty dwarf galaxy that was previously
reported with this dataset, which we conclude is due to a noise artefact. Thus,
the cause of the flux-ratio anomaly between the merging lensed images is still
unknown. The composite compact and diffuse emission in this system suggest
lensed quasar-starbursts will make excellent targets for detecting dark
sub-haloes and testing models for dark matter.
