600 million light years away! Scientists have discovered the nearest Einstein ring. What secrets are hidden behind it?

Einstein Ring

I saw a news report not long ago saying that "scientists have discovered the nearest Einstein ring." What is an "Einstein ring"? What is the use of discovering it?

In short, Einstein rings are a special astronomical phenomenon. Soon after Einstein proposed his general theory of relativity, he predicted that if a massive celestial body (such as a galaxy or a galaxy cluster) is located between the Earth and another luminous celestial body (such as a more distant galaxy), and the three are almost precisely aligned, then the gravitational field of the massive celestial body will bend the light emitted by the distant light source. This effect is called the "gravitational lensing" effect.

Depending on the scale and effect, gravitational lenses can be divided into microlensing, weak gravitational lensing, and strong gravitational lensing. Microlensing is generally caused by stellar objects, and the light deflection is very weak. Normally, the image formed by the lens cannot be directly observed, and only the brightness of the celestial body can be seen to be instantly enhanced. Weak gravitational lensing is more obvious because it is larger in scale and is usually caused by an entire galaxy. The most obvious one is strong gravitational lensing, whose foreground object is generally a huge galaxy cluster (at least a large elliptical galaxy). These celestial bodies are so massive that they are enough to severely distort passing light and even create multiple images of background light sources.

For example, the famous "Einstein Cross" is the four images of the background light source around the foreground object, which look like a "cross" when connected. Therefore, it is called the "Einstein Cross". If the mass distribution of the foreground object is relatively uniform and highly symmetrical, and the background light source is relatively large (not a small light spot, but a larger light spot), then the strong gravitational lens can even turn the background light spot directly into a ring, which is the "Einstein ring".

Although these phenomena were predicted a hundred years ago, people at that time were unable to verify them through observation due to technological limitations. It was not until later, with the development of radio astronomy, that astronomers discovered the first Einstein ring (MG1131+0456) through the Very Large Antenna Array (VLA) in 1988. However, because it was discovered through radio, its authenticity was not fully confirmed at the time. Later, with the advancement of optical telescopes, people finally discovered Einstein rings in the optical band. For example, B1938+666, discovered by the Hubble telescope in 1998, is considered to be the first clear discovery of an Einstein ring. With the help of the Hubble telescope, astronomers later discovered multiple Einstein rings.

Nowadays, we actually have telescopes that are better at detecting gravitational lenses, such as the Euclid telescope that was just launched in 2023. Not long ago, astronomers accidentally discovered a special Einstein ring in Euclid's data. The special thing is that the previously discovered rings are almost all over billions of light years away, but the newly discovered ring is only 600 million light years away from us (z=0.042), which is the closest to us so far.

Note: The distance of "600 million light-years" actually refers to the distance of the foreground object (that is, the distance from the elliptical galaxy NGC 6505 to us), and the real object in the distorted image is actually a more distant galaxy behind the elliptical galaxy, 4.42 billion light-years away from us (z=0.406). It can be seen that if it were not for the "magnifying glass" in front (NGC 6505), we might never find the galaxy behind it. This is also a use of Einstein rings and gravitational lensing in astronomy - it can magnify those very distant celestial bodies, allowing us to see and see more details, which is crucial for us to study the formation of stars in the early universe and the evolution of galaxies.

This article is a work supported by the Science Popularization China Creation Cultivation Program

Author: Linvo Talks About the Universe

Reviewer: Gou Lijun, researcher at the National Astronomical Observatory of the Chinese Academy of Sciences

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.