The closest ever Einstein ring, picked up by the European Space Agency’s Euclid space telescope
ESA
Astronomers have identified the closest ever Einstein ring, a rare phenomenon where light from a further-off galaxy is bent by the gravity of a galaxy closer to Earth. The ring was previously thought to be one galaxy and was identified more than 100 years ago.
Galactic lenses like this one, which is the closest astronomers have ever found, were predicted by Albert Einstein in 1936 from his theory of general relativity. At the time, he thought such an effect would be impossible to observe. In fact, he would have been able to see one if he had just had a powerful enough telescope. “It was there all along, but we had no idea,” says Thomas Collett at the University of Portsmouth, UK.
Collett and his team realised that the oval-shaped galaxy NGC 6505, which is about 600 million light years from Earth and was first spotted in 1884, was actually bending the light of a second galaxy behind it, about 6 billion light years from Earth.
A close-up of the Einstein ring
ESA/Euclid/Euclid Consortium/NASA, image processing by J.-C. Cuillandre, T. Li
Team member Bruno Altieri at the European Space Agency observed the Einstein ring while he was validating early testing data from the Euclid telescope, which has recently started scanning billions of galaxies over an area that will eventually span a third of the night sky. “There was this abundantly obvious Einstein ring. There’s not that many things in the universe that can produce a ring like this,” says Collett.
“We would have expected about a 1 in 3 chance of finding something as spectacular as this over the whole survey,” he says. “To find it in essentially the first, earliest data is spectacularly lucky. This is probably the prettiest lens we will find in the mission.”
The ring itself is exceptionally bright compared with most Einstein rings that we know of, says Collett, in part because it is very close to us, but also due to Euclid’s imaging capabilities. “It’s like someone with bad eyesight putting their glasses on,” says Collett. This makes it easier to see the four images of the faraway galaxy. The faint orange light surrounding the bright ring is the lensing galaxy.
Having an Einstein ring so close to Earth will let us test relativity in ways we can’t with other faraway lenses, says Collett, because they can measure the galaxy’s mass in two ways: using the amount the light bends and the speed of the stars, which are often too distant to accurately measure. Einstein’s general relativity says these masses should be the same, so any difference might suggest our theory of gravity needs to be modified.
When Collett and his colleagues measured the mass of the lensed galaxy, they also found a slightly higher number than should be possible from just the estimated number of stars in the galaxy. This could be due to dark matter clumping together in the galaxy’s centre, says Frédéric Dux at the European Southern Observatory, though we will need to find many more Einstein lenses to confirm, he says.
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