Enlarge / A radio image of MG 1131+0456, the first observed Einstein ring, taken with the Very Large Array network of radio telescopes.


Astronomers around the world may have lost access to their telescopes during the coronavirus pandemic, sheltering in place along with the rest of us, but that hasn’t kept them from advancing their field. Two astronomers used the shutdown to comb through existing datasets to hunt for a rare type of quasar and wound up rediscovering a so-called “Einstein ring” first observed back in 1987. They became the first to officially measure its distance from Earth, as reported in a recent paper published in The Astrophysical Journal Letters.

An Einstein ring is a direct consequence of the general theory of relativity; mass bends and warps spacetime, and light must follow that curvature. (An Einstein cross is an even rarer effect.) As Matthew Francis reported for Ars back in 2012:

For a sufficiently large mass, the light’s shift may be sufficiently large that we can measure it, and it can produce lensed images of the original light source. In gravitational lensing, the lens is a galaxy or galaxy cluster lying between Earth and a distant source, itself typically a galaxy. If the lens is directly in the line of sight, the image of the source galaxy can be distorted into an Einstein ring, a circular image of the source. By studying the shape and other characteristics of the image, observers can reconstruct details about both the lens and the source galaxies.

Einstein himself thought an Einstein ring would be impossible to observe, but he was thinking of rings formed by stars, noting that it would be highly improbable to get stars to align in just the right way to produce the “halo” effect. A single star would also form too small of a lens, thereby defying, as Einstein observed in a 1936 paper, “the resolving power of our instruments.” (The angular size of an Einstein ring increases with the mass of the lens.) But galaxies (and galaxy clusters) do make for a sufficiently massive lens.

Quasars are a type of active galactic nuclei—compact regions near a galaxy’s center that give off huge amounts of energy because the supermassive black holes at their centers are greedily devouring matter. They were first discovered by astronomers in the 1950s. It can be challenging to spot quasars because they are so distant, but gravitational lensing can help, since another galaxy closer to Earth can act as the lens, warping the light of the quasar behind it and making it brighter.

There are some 200 known gravitationally lensed quasars, but others are even harder to spot because they are obscured by all the gas and dust stirred up by the black hole as it feeds. Those were the objects of interest for Daniel Stern of NASA’s Jet Propulsion Laboratory and his co-author, Dominic Walton of the University of Cambridge’s Institute of Astronomy.

They combed through old public data collected by the W.M. Keck Observatory, as well as by NASA’s Wide-field Infrared Survey Explorer (WISE) and the Chandra X-ray Observatory. The WISE data in particular proved useful, since the gas and dust that obscures these objects when hunting for them using visible light are much easier to spot in infrared surveys.

Lurking in that data was the very first Einstein ring discovered by astronomers in 1987, known as MG 1131+0456, using the Very Large Array radio telescope network. “As we dug deeper, we were surprised that such a famous and bright source never had a distance measured for it,” said Stern. “Having a distance is a necessary first step for all sorts of additional studies, such as using the lens as a tool to measure the expansion history of the Universe and as a probe for dark matter.”

To rectify that oversight, Stern and Walton noted that the Keck Observatory had made several observations of the quasar between 1997 and 2007. That enabled them to calculate the distance of the object: 10 billion light years from Earth. Next, they determined the galaxy’s mass and used the Chandra X-ray data from 2000 to figure out just how much gas and dust there is between Earth and the quasar near its center.

“Our next step is to find lensed quasars that are even more heavily obscured than MG 1131+0456,” said Walton. “Finding those needles is going to be even harder, but they’re out there waiting to be discovered. These cosmic gems can give us a deeper understanding of the Universe, including further insight into how supermassive black holes grow and influence their surroundings.” Among other followup studies, the James Webb Space Telescope, slated for launch in March 2021, could be used to study the dark matter in the lensing galaxy.

“This whole paper was a bit nostalgic for me, making me look at papers from the early days of my career, when I was still in graduate school,” Stern said. “The Berlin Wall was still up when this Einstein ring was first discovered, and all the data presented in our paper are from the last millennium.”

DOI: Astrophysical Journal Letters, 2020. 10.3847/2041-8213/ab922c  (About DOIs).

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