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Inoue Lab. Kindai Univeristy
Department of Physics

Research

Probing small-scale cosmic structures with gravitational lensing

Light from a distant quasar (QSO) can be gravitationally lensed by a foreground elliptical galaxy, creating quadruple images. The flux ratios of such quadruple images in some systems disagree with the prediction of best-fit lens models with a smooth potential. This problem is called "the flux ratio anomaly problem". In order to explain the anomaly, a presence of substructures within the lensing galaxies or additional structures (halo, void) along the line of sight has been proposed. Using 6 quadruple lens systems observed at mid-infrared band, which is free from either microlensing or scattering due to free electrons, our team has found that the main cause of the anomaly is structures in the line of sight rather than substructures in the lensing galaxies. The contribution from subhaloes that reside in a lensing galaxy is just ~30% of the total. Our findings give a way to probe the matter distribution in the universe on sub-galactic scales.

Schematic picture of gravitatoinal lensing.
A quadruply lensed quasar MG0414+0534 in near-infrared band (left, Falco et al.) and mid-infrared band (right, Minezaki et al.).

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.