An Einstein ring is an extremely rare type of gravitationally lensed object, first predicted by Albert Einstein’s theory of relativity. Gravitational lensing occurs when the immense gravity of a massive foreground object, such as a galaxy cluster or a black hole, bends spacetime around it; light emitted by more distant objects, such as galaxies or supernovae, passing through this curved spacetime also appears curved from our perspective on Earth.
This effect also magnifies the light from the object being lensed, like a magnifying glass, allowing astronomers to study distant objects in greater detail than is normally possible. Most objects that are gravitationally lensed form arcs or partial rings surrounding the foreground object. However, a true Einstein ring forms a complete circle around a closer object, which is only possible when the distant object, foreground object, and observer are perfectly aligned.
In a study published in the journal Nature Astronomy, the researchers discovered a new, eerily circular Einstein ring, called JWST-ER1, as part of the COSMOS-Web survey, a detailed map of more than 500,000 galaxies obtained during 200 hours of continuous observations by JWST.
The complete Einstein ring, JWST-ER1, is the most distant gravitationally lensed object ever discovered. NASA/James Webb Space Telescope/van Dokkum et al.
JWST-ER1 is made up of two parts: JWST-ER1g, a compact galaxy that acts as a foreground lensing object; and JWST-ER1r, light from a more distant galaxy that forms a glowing ring. JWST-ER1g is about 17 billion light-years from Earth, while JWST-ER1r is another 4 billion light-years further away. Until now, the farthest lensing object detected was about 14.7 billion light-years away, according to BigThink.com. (While the universe itself is estimated to be about 13.7 billion years old, the universe’s constant expansion means that light from the oldest objects must travel much further to reach our telescopes.)
Using the full ring of JWST-ER1, the researchers calculated the mass of the lensing galaxy by determining how much it warps the spacetime around it. This showed that the galaxy has a mass equivalent to about 650 billion suns, making it unusually dense for its size. Some of this extra mass could be explained by dark matter, the mysterious, invisible stuff that makes up about 85% of all matter in the universe. But even so, the researchers calculated that it’s unlikely that there are enough stars to account for the rest of the galaxy’s mass.
“Additional mass is needed to explain the lensing results,” but it is not yet clear what that mass is, the researchers write in their paper.
A collection of incomplete and nearly perfect Einstein rings photographed by NASA’s Hubble Space Telescope. To get a perfect circle, the background object, foreground object, and observer must be perfectly aligned. NASA
Other galaxies that are just as old and dense have been discovered before, suggesting that these ancient star factories have something in common that makes them so massive. One explanation is that these galaxies contain much more dark matter than previously thought, while another theory suggests that they may have more low-mass stars hiding inside them than younger galaxies. But more research is needed to find out.
This isn’t the first true Einstein ring discovered by the JWST. In September 2022, a Reddit user discovered a perfectly circular ring of light from the galaxy JO418, located about 12 billion light-years from Earth, lensed around a closer galaxy. The JWST also used gravitational lensing to capture the most distant star ever discovered and one of the oldest galaxies in the universe.
This stunning halo is the result of light from a distant galaxy passing through the warped spacetime surrounding another galaxy that is aligned between the distant light source and Earth. This 2022 James Webb Space Telescope image, created by an enthusiastic astronomer on Reddit, is one of the best examples of this unusual astronomical phenomenon ever captured.
The ring of light in the new image comes from the distant galaxy SPT-S J041839-4751.8 (or JO418 for short), which is about 12 billion light-years from Earth, making it one of the oldest galaxies in the universe. JO418 is oriented directly behind another galaxy — the bright blue light at the center of the ring — which is so massive that its gravitational pull curves spacetime around it. When the light from JO418 reaches the foreground galaxy, it passes through this curved spacetime. From Earth, it looks like the light has curved around the galaxy, but the electromagnetic waves we see have actually been traveling in a straight line the whole time.
Close up of Einstein ring JO418. Spaceguy44
This strange effect is similar to the way glass lenses redirect light. Like magnifying glasses, this phenomenon also makes light from distant galaxies appear much closer than they actually are. The only difference is that the lens is made of gravity-warped spacetime instead of glass. That’s why researchers call this strange effect gravitational lensing. Albert Einstein first predicted gravitational lensing in 1912 when he developed his theory of relativity.
Reddit user and graduate student astronomer Spaceguy44 posted an image of the JOS18 Einstein ring on August 23 to the r/Astronomy subreddit. The anonymous astronomer created the image using publicly available data from the James Webb Space Telescope’s Mid-Infrared Imager (MIRI) instrument.
“We wouldn’t be able to see J0418 if it weren’t for the bending of gravity,” Spaceguy44 wrote on Reddit. “Without the lensing effect, the galaxy would probably look like most distant galaxies: like a small blob of light.”
A nearly perfect Einstein ring from galaxy JO418. Spaceguy44
In 2020, researchers discovered a distant galaxy by detecting partial gravitational lensing using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile; they reported their discovery in a paper published that year in the journal Nature.
On August 13, Spaceguy44 posted an image of JO418 using data collected by Webb’s NIRCam instrument, but the original image had a much lower resolution and the ring of light was less visible, according to ScienceAlert.
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NASA’s Hubble Space Telescope has captured images of several other Einstein rings, including one formed from the bent light of a quasar. However, none of these Einstein rings were as complete or clear as the one in the Reddit image.
Perfectly round Einstein rings are extremely rare because they require both distant and nearby galaxies to be perfectly aligned with the observer. However, better sensors on the Webb telescope should make them easier to spot in the future.
A new analysis of an unusually massive but compact galaxy from the early Universe shows that dark matter interacts with itself.
Formed just 3.4 billion years after the Big Bang, JWST-ER1 was first discovered in October 2023 in images taken by NASA’s James Webb Space Telescope (JWST). More than 17 billion light-years from Earth, JWST-ER1g is the most distant example of a perfect “Einstein ring” — a continuous circle of light around a galaxy caused by the bending of light from a distant, unseen galaxy by the space-warping mass of JWST-ER1. The research was published online April 11, 2024, in The Astrophysical Journal Letters.
The cosmic mirage is not just a pretty sight created by the fortunate alignment of galaxies; it also provides physicists with a valuable probe for model-independent measurements of the mass contained within the ring’s radius.
By calculating how much JWST-ER1g warps spacetime around itself, the team concluded that the galaxy weighs about 650 billion suns, making it unusually dense for its size. By subtracting the visible stellar mass from the estimated total mass, the physicists can determine how much of the galaxy is made up of dark matter, the invisible substance thought to make up more than 80% of all matter in the universe.
The unusually dense galaxy JWST-ER1 and its Einstein ring, as imaged by the James Webb Space Telescope in 2023. P. van Dokkum et al.
Despite decades of observations and a wealth of indirect evidence, the elusive substance has yet to be directly detected. In JWST-ER1g, the team found that dark matter accounts for about half of the mass difference, and that “additional mass is likely needed to explain the lensing effect,” according to a paper published in 2023.
“The dark matter mass value appears to be higher than expected,” said Hai-Bo Yu, a professor of physics and astronomy at the University of California, Riverside (UCR) and a co-author of the new study. “It’s puzzling.”
In the paper, Yu and his colleagues suggest that JWST-ER1g’s unusually high density could be explained by a higher population of stars than currently thought. However, as lead study author Demao Kong of the University of California, Riverside, noted in the same statement, the compression mechanism by which ordinary matter — the stuff that makes up gas and stars — “collapses and condenses” into JWST-ER1g’s dark matter halo could result in “more dark matter packing into the same volume, resulting in a higher density.”
The dark matter halo, densest at the center of the galaxy, acts as a gravitational glue that keeps rotating galaxies from flying apart. What’s more, models that include a certain type of dark matter, whose particles interact with each other, “match the JWST-ER1 measurements very well,” according to the new study.
The James Webb Space Telescope’s mirrors are assembled. NASA/MSFC/David Higginbotham/Emmett
We don’t yet know what dark matter really is. Observational evidence suggests that it is a new type of particle, whose existence can only be inferred from its gravitational interactions with ordinary matter. Dark matter may be just one type of particle, or a complex collection of different types, as in ordinary matter, which may be acting in the presence of additional, previously unknown forces unique to dark matter.
In December 2023, Yu led a simulation of the formation of structures involving self-interacting dark matter, concluding that such self-interactions could explain the extremely dense dark matter halos in some galaxies, as well as the mysteriously low densities in others, both of which are not explained by the prevailing “cold dark matter” theory.
Physicists hope that JWST will be able to shed more light on dark matter, so to speak. The telescope’s unprecedented infrared capabilities allow it to peer further into the past than any other telescope, and upcoming studies of galaxies from the earliest universe could provide clues to dark matter particles and their behavior, Yu said.
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