Galaxies are like fingerprints, or snowflakes. There are a lot of them, and they may have many characteristics in common, but no two are exactly the same.
So, back in 2013, when two galaxies were spotted together in the far reaches of the universe, and which looked surprisingly similar, astronomers were shocked.
Now, they have finally solved the mystery of these strange “similar objects” – and the answer may have implications for understanding dark matter.
The object now named Hamilton’s object was discovered, by accident, by Shawnee State University astronomer Timothy Hamilton in data obtained by the Hubble Space Telescope nearly a decade ago.
The two galaxies appeared to be about the same size, and had nearly identical parallel dark streaks on the galactic bulge—the central region of the galaxy where most of the stars reside.
“We were really stumped,” Hamilton said. “My first thought was that maybe they were galaxies interacting with tidal arms. It didn’t really fit well, but I didn’t know what else to think.”
It was not until 2015 that a more plausible answer would emerge. Astronomer Richard Griffiths of the University of Hawaii, seeing Hamilton present his object at a meeting, suggests that the culprit may be a rare phenomenon: gravitational lensing.
It is a phenomenon that occurs purely as a result of coincident alignment of massive objects in space. If a massive object sits directly between us and a more distant object, the magnification effect is due to the gravitational curvature of space-time around the nearer object.
Any light that travels through this space-time follows this curvature and enters our telescopes which are distorted and distorted to varying degrees – but often magnified and repeated.
This makes a lot more sense than two identical galaxies, especially when Griffith found another duplication of the Milky Way (as can be seen in the photo below).
However, a major problem remained: what was causing the gravitational curvature? So Griffith and his team set about finding sky survey data for an object large enough to produce the lensing effect.
And he found it. Between us and Hamilton’s object lies a cluster of galaxies that was only poorly documented. Usually, these discoveries go the other way – first the cluster is identified, and then astronomers go looking for galaxies with lenses behind them.
The team’s work showed that Hamilton’s object is about 11 billion light-years away, and work from a different team showed that the cluster is about 7 billion light-years away.
The researchers determined that the galaxy itself is a barred spiral galaxy, with the edge facing us, undergoing strange and uneven star formation. Computer simulations then helped to determine that the three duplicate images could only be created if the distribution of dark matter was smooth at small scales.
“It’s great that we only need two mirror images to get an idea of how viscous or dark matter may not be at these positions,” said astronomer Jenny Wagner of Heidelberg University in Germany.
“Here, we don’t use any lens model. We just take the observables of several images and the fact that they can be transformed into each other. They can be folded into each other with our method This already gives us an idea of how smooth dark matter must be in these two conditions.”
The two identical side-by-side images were created because they spanned a “wave” in space-time—the region of greatest magnification created by the gravity of a filament of dark matter. Such fibers are believed to connect the universe into a vast, invisible cosmic web, joining galaxies and galaxy clusters and feeding them hydrogen gas.
But we don’t really know what dark matter is, so any new discoveries that tell us exactly where it is, how it’s distributed, and how it affects the space around it will be more than enough evidence. There is one more drop that will eventually help us solve the mystery.
“We know it’s some kind of matter, but we don’t know what the constituent particle is,” Griffiths explained.
“So we don’t know how it behaves. We just know that it has mass and is subject to gravity. The importance of the size limit on clumping or smoothness is that it gives us some clues as to what the particle is. The smaller the dark matter, the larger the particles must be.”
research has been published in Monthly Notice of the Royal Astronomical Society.