Mystery of a strange, mirrored double galaxy solved using Hubble data

When telescopes like Hubble peer out into the cosmos, they can sometimes sneak a view of very distant objects using a phenomenon called gravitational lensing. This is where a massive object like a galaxy passes between a distant object and Earth, and the gravity of this intermediate object warps spacetime and acts as a magnifying glass. This allows astronomers to glimpse objects which would otherwise be too faint and far away to be visible.

But sometimes these phenomena result in strange outcomes, like an oddity spotted by Hubble in 2013 which appeared to be two objects that were perfect mirror images of each other.

This Hubble Space Telescope snapshot shows three magnified images of a distant galaxy embedded in a cluster of galaxies.
This Hubble Space Telescope snapshot shows three magnified images of a distant galaxy embedded in a cluster of galaxies. LEAD AUTHOR: NASA, ESA, Richard E. Griffiths (UH Hilo) CO-AUTHOR: Jenny Wagner (ZAH) IMAGE PROCESSING: Joseph DePasquale (STScI)

Astronomers are used to seeing galaxies that appear to be stretched into odd shapes due to gravitational lensing, but this mirroring effect was baffling. “We were really stumped,” said astronomer Timothy Hamilton of Shawnee State University, who first spotted the object, in a statement.

Since then, Hamilton and others have been studying the strange object and have finally figured out its puzzling nature. They found that there was a massive cluster of galaxies that was previously uncataloged between Earth and the object, aligned in such a way that it produced two twin images of the distant galaxy. The background galaxy stretches across a ripple in space created by dark matter — and this ripple creates not only the two mirrored images but also a third image of the galaxy positioned 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,” explained another of the researchers, Richard Griffiths of the University of Hawaii in Hilo. “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.”

This very rare finding doesn’t only let astronomers sneak a peek at a distant galaxy — it can also help them learn about the dark matter which makes up a large percentage of our universe, by showing how “clumpy” or “smooth” it is. That’s important as there’s a lot we are yet to understand about this mysterious matter.

“We know it’s some form of matter, but we have no idea what the constituent particle is,” Griffiths said. “So we don’t know how it behaves at all. We just know that it has mass and is subject to gravity. The significance of the limits of size on the clumping or smoothness is that it gives us some clues as to what the particle might be. The smaller the dark matter clumps, the more massive the particles must be.”

The research is published in the journal The Monthly Notices of the Royal Astronomical Society.

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