Research Highlight

Weighing the mass of dark matter using gravitational lensing

When a celestial object (such as a galaxy) lies along the line-of-sight to a more distant object (such as a quasar or QSO), the gravitational field of the former refracts the light of the latter,  resulting in deformation or splitting of light into multiple images (called 'gravitational lensing'). By analyzing gravitationally lensed images, we can study the dark matter distribution in the line-of-sight as well as that of lensing galaxies. If the dark matter particles have a large velocity dispersion at the time of decoupling from a thermal bath(plasma), the structure of dark matter produced by gravitational force can be erased since the dark matter particles freely stream. As the decoupling time of lighter dark matter particles is much delayed in comparison with heavier ones, the distance that particles can travel freely  becomes larger for lighter particles. This means that the erasing effect is larger for lighter particles.  Thus from the structures of dark matter in the line-of-sight, we can put a lower limit on the mass of dark matter.  Using observed 6 four-image gravitational lenses, and high-resolution N-body simulations, we obtained a lower limit on the mass of dark matter particles  (mass>= 1.3keV(95% confidence level) ).  In other words, we found that the mass of dark matter particles in the universe should be larger than 1/390 of the electron mass.    (see 2015 MNRAS, Vol. 448, issue 3, p2704-2716).

 

A distant QSO is split into multiple images due to gravitational lensing.

Structures in the line-of-sight (voids/halos) and subhalos affect the magnification and distortion of lensed images.