Research Highlight

On the origin of flux-ratio anomaly

Gravitational lens is a powerful tool for exploring the nature of misterious dark matter in the universe.  Clumps and voids of dark matter can act as convex and concave lenses. For instance, a dark matter clump that host a galaxy is called a 'halo' , which can act as a convex lens. If a halo has an elliptic symmetry, then it can produce 3 or 5 multiply lensed images. However, it is difficult to find a 'fifth' image that is very close to the center of the lens as it is too dim.  Therefore, we usually observe lens systems with double or quadruple images. It has been known that some quadruply lensed systems show anomalies in the observed flux ratios of lensed images: the positions of images can be fitted but the flux ratios cannot by using a smooth lens.  Until recently, the origin of such anomaly has been attributed to 'subhalos', i.e., small mass halos inside a larger mass halo that can act as a primary lens. However, such anomaly can be also caused by dark matter clumps/voids in the line-of-sight to the light source.  Using 11 quadruply lensed systems,  I found that the density of dark matter necessary to explain the anomaly has a positive correlation with the distance of the light source (QSO).  This finding can be naturally explained by line-of-sight dark matter as the dominant source of the anomaly. I have also found that the contribution from 'subhalos' is just 20-40 percent; it is subdominant. The result was published in Monthly Notices of Royal Astronomical Society, 31 May 2016.

 

 

 

Amplitude of surface mass density of dark matter versus source redshift for five anomalous lenses.

The red circles are observed values. The blue rhombuses and circles are expected contributions

from line-of-sight dark matter fluctuations and subhalos, respectively.

The ratio of expected lensing contributions from subhaloes to the total (subhaloes+line-of-sight haloes/voids)

 in 11 quadruple lens systems (blue circles) in a concordant LCDM model.  z_s is the redshift of a

source QSO.  The dotted line represents the best-fit linear fit to the values of the 11 samples.  The typical

subhalo contribution is ~30 percent. The remaining 70 percent is due to structures (voids/haloes) in the

intergalactic space between a source and an observer.