Galaxies are somewhat like fingerprints or snowflakes. There are a lot of them, and they can have a lot in common, but no two are exactly alike.
So, in 2013, when two galaxies were spotted side by side in the far reaches of the universe that looked astoundingly similar, astronomers were confused.
The object, now called the Hamilton Object, was discovered by astronomer Timothy Hamilton of Shawnee State University by accident, in data obtained by the Hubble Space Telescope nearly a decade ago.
The two galaxies appeared to be the same shape, with roughly the same parallel dark stripes across the galactic bulge – the central region of the galaxy where most stars live.
“We were so confused,” Hamilton said. “My first thought was that they might have been interacting with galaxies with outstretched arms. It wasn’t really convenient, but I didn’t know what to think about either.”
A more reasonable answer didn’t emerge until 2015. Astronomer Richard Griffiths of the University of Hawaii, upon seeing Hamilton present his body at a meeting, suggested that the culprit might be a rare phenomenon: gravitational lensing.
This is a phenomenon entirely caused by the accidental alignment of massive objects in space. If a massive object lies directly between us and a farther object, the magnification effect is caused by the gravitational curvature of spacetime around the nearest object.
Any light that then travels through this spacetime that follows this curvature and enters our telescopes is smeared and distorted to varying degrees – but also often magnified and doubled.
This made more sense than two identical galaxies, especially when Griffiths found another galaxy duplicate (as can be seen in the image below).
However, a major problem remained: What caused the curvature of gravity? So Griffiths and his team set out to search sky-scan data for an object large enough to produce a lensing effect.
And they found it. Between us and Hamilton’s body lurks a cluster of galaxies that have only been poorly documented. Usually such discoveries go the other way – first the mass is determined, then astronomers look for galaxies with a lens behind it.
The team’s work revealed that the Hamilton object is about 11 billion light-years away, and the work of a different team revealed that the cluster is about 7 billion light-years away.
The researchers determined that the galaxy itself is a spiral galaxy with bars whose edge faces us, and undergoes a lumpy and uneven star formation. Computer simulations then helped determine that the three duplicate images could only be generated if the dark matter distribution was smooth on small scales.
“It’s great that we only need two mirror images in order to get a measure of how lumpy dark matter is or not in these positions,” Astronomer Jenny Wagner said: from Heidelberg University in Germany.
“Here, we’re not using any lens models. We’re just taking the notes from the multiple images and the fact that they can be turned into each other. They can be folded into each other by our method. This gives us an idea of how smooth the dark matter is at these two positions.”
The two identical images were created side by side because they span a ‘ripple’ in spacetime – a region of greater magnification caused by the gravity of a filament of dark matter. It is believed that these threads connect the universe to a vast and invisible cosmic web, join galaxies and galaxy clusters and feed them with hydrogen gas.
But we don’t actually know what dark matter is, so any new discovery that allows us to determine where it is, how it is distributed and how it affects the space around it is another point of evidence that will eventually help us solve the mystery.
“We know it’s a form of matter, but we have no idea what the particle is made of,” Griffith explained.
“So we don’t know how it behaves at all. We just know that it has mass and is subject to gravity. The importance of size limits over agglomeration or smoothness is that it gives us some clues as to what a particle might be. The smaller the dark matter, the more particles should be Mass “.
The search was published in Monthly Notices of the Royal Astronomical Society.