A new result from Dr. LI Rui and Prof. WANG Jiancheng (from Yunnan Observatories, Chinese Academy of Sciences) has been published on line by Monthly Notices of the Royal Astronomical Society. They analyzed a modified gravity (Rastall gravity) and then observationally constrained the parameters in this gravity with galaxy-scale gravitational lensing.

Rastall gravity is a kind of modified General Relativity that assumes the energy-momentum is not conserved in curved space-time. The energy–momentum conservation law is one of the basic elements in General Relativity, people have only observationally checked it in flat space–times, but it is maybe not the case in curved space–time.

In order to release the energy–momentum conservation law, Rastall gravity was proposed. It introduces a new interaction between the geometry and the matter field. There are many applications in cosmology and black-hole space–time for Rastall gravity recently. But in galaxies, there are no applications and observational constraints to the parameters in Rastall gravity.

For elliptical galaxies, their mass density is approximately a power-law form, which has not been explanted by General Relativity. LI Rui and WANG Jiancheng found that this power-law form is a natural result of the Rastall gravity and the power-law index is determined by two parameters in Rastall gravity.

How to obtain the power-law index for the mass density of elliptical galaxies? Researchers found that it could be inferred from strong gravitational lensing.

It is known that the light can be blended by massive objects. If a source galaxy is located behind a foreground galaxy, its light would be blended by the gravity of the foreground galaxy.

Therefore, around the foreground galaxy, several images of the source galaxy could be found. This phenomenon is known as galaxy–galaxy strong gravitational lensing. The shapes and locations of the images of the background galaxy are determined by the mass and mass distribution of the foreground lens.

With gravitational lensing, combined with galactic dynamics, the power-law index for the mass density could be obtained. Then it could be used to constrain the parameters in Rastall gravity and judge the rationality of this modified gravity.

“The research provides a new way to prove the gravity theory”, Professor WANG said.

Contact:

LI Rui

Yunnan Observatories, CAS

lirui@ynao.ac.cn