An odd image of a distant galaxy has revealed what might be an enormous blob of dark matter hiding in plain sight.
When astronomers first looked at the new image of the galaxy HerS-3, taken by the Northern Extended Millimeter Array radio telescopes in France, they thought there was an error in the data.
The image showed what appeared to be an "Einstein cross." This rare phenomenon occurs when light from a distant galaxy or quasar (a bright galaxy with an actively-feeding black hole at its center) bends around the gravity of a massive object in front of it, so that it appears to have split into four points from the perspective of an observer. But what made this configuration so unusual was the fifth point of light glowing in its center. At first, "we thought it was a problem with the instrument," Cox said.
Because the photons of an Einstein cross curve around a central mass, scientists wouldn't normally expect to see a fifth point in the middle.
"You can't get a fifth image in the center unless something unusual is going on with the mass that's bending the light," Charles Keeton, an astronomer at Rutgers University and co-author of a new study describing the findings, said in a statement.
In the study, published Sept. 16 in The Astrophysical Journal, the researchers used computer modeling to try to figure out exactly what was going on with the weird cross. Their analysis revealed that all of the points of light originated from HerS-3, which ruled out the possibility that a closer, brighter object was getting in the way. They also ruled out a straightforward instrument malfunction by checking the image against data collected by the Large Millimeter/submillimeter Array (ALMA) in Chile.
Finally, they ran a computer simulation in which a mass of dark matter sat in front of HerS-3 — and this time, the results matched their observations.
Dark matter is notoriously difficult to image. It does not absorb, reflect or emit light, so it's functionally invisible. However, it does have gravity. A large halo of dark matter would have enough gravitational pull to bend the light of a galaxy directly behind it while leaving the galaxy itself visible (and therefore capable of being warped into an Einstein cross).
The discovery offers an exciting opportunity for scientists to learn more about how dark matter interacts with other cosmic objects. "We can study both the distant galaxy and the invisible matter that's bending its light," Cox said.
The team hopes to use these future observations to further test and refine the computational models.
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