A chance configuration of objects arrayed across deep space has just revealed the hiding place of a giant glob of dark matter.
Configurations like these, known as Einstein crosses, typically consist of four distinct points of light. This particular example, named HerS-3, has a feature never seen before. At the center of the cross appears a fifth blob of light.
“That’s not supposed to happen,” says theoretical astrophysicist Charles Keeton of Rutgers University-New Brunswick in the US. “You can’t get a fifth image in the center unless something unusual is going on with the mass that’s bending the light.”
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An Einstein cross in and of itself is a particularly rare cosmic phenomenon, created by light traveling through spacetime warped by the presence of an immense field of gravity. When light from a distant object, such as a galaxy, travels through this curved spacetime, it can split into four images of the galaxy that produced it, like the points of a cross.
Because the light is curved around the central mass, you don’t see a fifth image of the background object in the center of the cross; if there is a light in the center, it’s usually something in the foreground.
HerS-3 is a dusty, star-forming galaxy close to the edge of the visible Universe, emitting light that has traveled for 11.7 billion years to reach us.
Even at first glance, it seemed unusual. When a team led by astronomer Pierre Cox of the French National Center for Scientific Research (CNRS) took observations of it, they found their instincts correct: the light from the central dot was coming from the same distance as the four dots around it.
“We were like, ‘What the heck?’” Cox says. “It looked like a cross, and there was this image in the center. I knew I had never seen that before.”
To find out what was causing the strange image, the researchers ran through a gamut of possible explanations. Initially, they thought it was a glitch, but it turned out to be quite real. Computer modeling also ruled out any of the foreground galaxies as an explanation for the peculiar lensing.
Eventually, they could only conclude that the mechanism behind the warped region of spacetime had to be something we can’t actually see: dark matter.
“We tried every reasonable configuration using just the visible galaxies, and none of them worked,” says Keeton. “The only way to make the math and the physics line up was to add a dark matter halo. That’s the power of modeling. It helps reveal what you can’t see.”
Dark matter is one of the Universe’s great unknowns – a mysterious form of matter that only interacts with the rest of the Universe via gravity. We know it exists because there’s way more gravity in the Universe than normal matter can account for. The Einstein lensing of HerS-3 is a case in point.
The researchers’ modeling suggests that a closer group of galaxies whose light has traveled for about 8 billion years combines with a massive clump of dark matter, or dark matter halo, to produce the observed Einstein ring.
It’s a remarkable find. That chance blob of dark matter sitting between us and HerS-3 magnifies the distant galaxy, giving us a much closer view of an active star-forming object in an early epoch of the Universe in which galaxies are usually too faint to resolve. It also offers a means of studying the nearer galaxy group, as well as the dark matter halo itself.
“The HerS-3 system with its exceptional Einstein cross with a fifth central image has been revealed by the present study to be a unique astrophysical laboratory to explore, at small spatial scales, a nearly edge-on dusty starburst galaxy at the peak of cosmic evolution and, importantly, to study the characteristics of the galaxy group lensing HerS-3 and the properties of the associated massive dark matter halo,” the researchers write in their paper.
The research has been published in The Astrophysical Journal.