Mystery of ‘double galaxy’ that stumped scientists for years finally solved

The strange objects were eventually found to be stretched images of a distant galaxy more than 11 billion light-years away

Adam Smith
Friday 08 October 2021 13:12
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Astronomers have revealed the truth about a mysterious ‘double’ galaxy that left them “stumped” when it was first discovered.

Nasa’s Hubble Space telescope discovered the two galaxy bulges that were mirror images of each other, with another strange object nearby.

"We were really stumped," said astronomer Timothy Hamilton of Shawnee State University in Portsmouth, Ohio.

The linear objects were eventually found to be stretched images of a distant galaxy more than 11 billion light-years away.

The images were warped because of a previously-undiscovered cluster of galaxies that were magnifying, brightening, and stretching the image of the galaxy behind it (known as gravitational lensing) due to their immense mass and the subsequent effect on spacetime.

In this specific, rare, case, the precise alignment between the background and foreground galaxy produced twin magnified copies of the same image – with another, third, image to one side.

"Think of the rippled surface of a swimming pool on a sunny day, showing patterns of bright light on the bottom of the pool," Richard Griffiths of the University of Hawaii explains.

"These bright patterns on the bottom are caused by a similar kind of effect as gravitational lensing. The ripples on the surface act as partial lenses and focus sunlight into bright squiggly patterns on the bottom."

When Hamilton first discovered these objects in 2013, it was unclear what they were. "My first thought was that maybe they were interacting galaxies with tidally stretched-out arms," Hamilton said. "It didn’t really fit well, but I didn’t know what else to think."

Eventually, the scientists discovered the strange cluster of galaxies causing the magnification and determined the distance of the strange objects – which was eventually discovered to be the same galaxy – using spectroscopic measurements from the Gemini and W. M. Keck observatories.

Using specialised computer software, they found that the dark matter around the stretched images had to be ‘smoothly’ distributed in space at small scales. "It’s great that we only need two mirror images in order to get the scale of how clumpy or not dark matter can be at these positions," Jenny Wagner of the University of Heidelberg in Germany, and an expert in gravitational lensing theory, said.

"Here, we don’t use any lens models. We just take the observables of the multiple images and the fact they can be transformed into one another. They can be folded into one another by our method. This already gives us an idea of how smooth the dark matter needs to be at these two positions."

Astronomers still do not know what dark matter is, even though it has been nearly 100 years since its discovery, but the “significance of the limits of size on the clumping or smoothness is that it gives us some clues as to what the [constituent] particle [of dark matter] might be”, Griffiths said, adding that “the smaller the dark matter clumps, the more massive the particles must be.”

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