What is a quasar flux ratio anomaly? In the context of strongly lensed, multiply-imaged quasar observations, I sometimes hear about "flux ratio anomalies". What are these? Why are they important?
 A: A massive object (such as a galaxy) along the line of sight to a distant bright source (such as a quasar) bends the light along its path. If the "lensing" object is massive enough and the geometry is right, the background object can be seen as multiple sources. For instance, here is a galaxy (central point) and four images of a single quasar:

For a well-behaved, nicely symmetric lens the positions of the images yield a strong prediction for the relative flux (brightness) of each image. The images lie on a ring (with a radius called the "Einstein radius"). For example, if the top and bottom images appeared closer to the left-hand image in the picture above, the right-hand image would be relatively fainter. Systems that do not obey the predicted flux ratios are said to exhibit a flux ratio anomaly.
A flux ratio could have a number of causes:


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*Dust or other faint/diffuse structure along the line of sight to one of the images (but not the others) causing one of the images to dim.

*If the background source (quasar) is time-varying, the flux ratios may be different because of the different light travel times for the different images. In other words, the same background quasar is seen simultaneously at different times.

*Some asymmetric structure in the lens, or additional structure along the line of sight, can break the symmetry that yielded the flux ratio predictions in the first place.


This last possibility is of considerable interest, especially if the "substructure" perturbing the lens (or along the line of sight) is a low-mass dark matter halo ($\lesssim 10^{7}\,{\rm M}_\odot$). The existence of these halos is a fundamental prediction of the standard $\Lambda{\rm CDM}$ theory of cosmology. Conversely, such low mass halos are not expected to exist at all in $7\,{\rm keV}$ sterile neutrino warm dark matter models, the currently preferred "flavor" of warm dark matter, except perhaps for systems undergoing stripping by a more massive nearby structure. A detection, or strongly constrained non-detection, of such a low mass dark matter halo is therefore a strong test of cosmology.
