When gravity—an invisible yet ubiquitous force—bends and distorts light from distant galaxies, Earthbound observers get a rare glimpse of elusive cosmic phenomena. And the weirder the glitch, the more insightful the finding, as a team of astronomers recently discovered.
These glitches are formally referred to as Einstein Crosses. As light from a distant galaxy travels toward our observatories, the gravitational force of galaxies closer to us bends that light, creating an image of four bright dots arranged like a cross. But astronomers spotted an Einstein Cross with an additional fifth image they couldn’t explain.
Further analysis revealed that a “massive dark matter halo” likely caused this unusual cosmic pattern, the researchers report. Their findings were published on September 16 in The Astrophysical Journal.
Not a glitch in the data
As with many great discoveries in astronomy, the team was initially studying something else. In this case, the researchers had the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile trained on HerS-3, a dusty galaxy located 11.6 billion light-years from Earth.
Everything looked normal until a pesky fifth object kept appearing in their data. At first, they suspected a technical issue on their end. But no matter how hard they tried, the fifth image wouldn’t go away.
The left panel shows the galaxy HerS-3, which is gravitationally amplified in an Einstein cross with a bright fifth central image. The yellow star indicates the position of the dark matter (DM) halo associated with the group. Credit: Cox et al./ALMA (ESO/NAOJ/NRAO)/NOEMA“We were like, ‘What the heck?’” Pierre Cox, study lead author and research director at the French National Centre for Scientific Research, recalled in a release. “It looked like a cross, and there was this image in the center. “I knew I had never seen that before.”
“I said, ‘Well, that’s not supposed to happen,’” added Charles Keeton, study co-author and an astrophysicist at Rutgers University, in the same release. “You can’t get a fifth image in the center unless something unusual is going on with the mass that’s bending the light.”
Science takes a dark turn
Later, Keeton and his colleagues ran computer simulations of the gravitational lensing used to capture this five-pronged Einstein Cross. They were baffled to find that no “reasonable configuration using just the [four] visible galaxies” fit the data.
Unless the configuration was hiding some large, invisible fifth mass, the image just didn’t make sense. So they did exactly that—they added some invisible stuff.
An infographic showing the mechanisms behind the observation of an Einstein Cross. Credit: N. Lira/Cox et al./ALMA (ESO/NAOJ/NRAO)Specifically, the researchers incorporated dark matter—the “missing” mass supposedly making up 85% of the universe. Plenty of evidence suggests that dark matter exists, though we only notice it through the way it influences visible objects around it.
Once dark matter—in this case, a halo weighing several trillion times the Sun’s mass—was factored in, the math and physics lined up beautifully, Keeton explained. “That’s the power of modeling,” he added. “It helps reveal what you can’t see.”
A ‘natural laboratory’
With the confusion cleared up, the researchers began investigating the Einstein Cross in earnest. Gravitational lensing offers a rare view of both the background galaxy and the dark matter surrounding the foreground galaxies, they explained.
“This system is like a natural laboratory,” Cox said. “We can study both the distant galaxy and the invisible matter that’s bending its light.”
For the moment, the team has made several predictions about the physics of the galaxies involved, including their shape and kinematics. They also hope to pinpoint quantifiable limits on the dark matter halo and “any associated stellar emission” to “show evidence of substructures,” according to the paper.
Readers of Gizmodo’s space coverage may remember my deep appreciation for gravitational lensing. This latest discovery shows why: the technique holds enormous potential for uncovering the universe’s greatest mysteries, many of which remain hidden from view.
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