Galaxies can be compared to fingerprints or snowflakes. Although there are many, they all have their own unique characteristics.
Astronomers were baffled when, in 2013, two galaxies were seen side-by-side in distant reaches. They looked startlingly alike.
They have now solved the mystery of the strange "identical objects", and this could have implications for understanding dark matter.
Hamilton's Object is now the name of the object. It was found by Timothy Hamilton, an astronomer at Shawnee State University, by accident using data from the Hubble Space Telescope almost a decade ago.
Both galaxies looked almost identical and shared the same dark streaks that ran across the galactic bulge, the region in which most stars reside.
Hamilton stated that Hamilton was "really stumped". Hamilton said that his first thought was that they might be interfacing galaxies with tidally stretched arms. Although it didn't fit very well, I wasn't sure what else to believe.
A more plausible explanation was only discovered in 2015. After Hamilton presented his object at a meeting of the University of Hawaii Astronomer Richard Griffiths suggested that it might be gravitational lensing, a rare phenomenon.
This phenomenon is caused by the chance alignment of large objects in space. Magnification occurs when a large object is placed directly between us and another object. This happens because of the gravitational curvature space-time around the distant object.
Any light that travels through this space time follows the curvature. It enters our telescopes smeared, distorted, magnified, and sometimes duplicated.
Illustration of gravitational lensesing. (NASA, ESA & L. Calada)
This was a lot more sensible than two identical galaxies. Especially when Griffiths discovered another duplicate of the galaxy (as you can see in the image below).
But there was one problem: What was causing gravitational curvature. Griffiths and his team began searching sky survey data in search of an object large enough to cause the lensing effect.
They found it. A cluster of galaxies lies between us and Hamilton's object. It was only poorly documented. These discoveries usually go in the opposite direction. First, the cluster is identified and then astronomers look for lensed galaxies.
Hamilton's Object lies around 11 billion light years away. A different team discovered that the cluster lies about 7 billion light years away.
Researchers discovered that the galaxy is a barred spiral galaxy, with its edge facing us. It also undergoes uneven and clumpy star formation. Computer simulations revealed that the creation of the three duplicated images was possible only if dark matter distribution is uniform at small scales.
Jenny Wagner, an astronomer at the University of Heidelberg, Germany said that "it's amazing that we only require two mirror images to get the scale of just how clumpy and not dark matter can exist at these positions."
"Here, there are no lens models. We only look at the observables in multiple images, and how they can be combined. Our method allows them to be folded together. This gives us an indication of how smooth the dark matter must be in these positions.
These two identical images, side-by-side, were created because they cross a "ripple", a region of maximum magnification caused by gravity from a filament dark matter. These filaments are believed to link the Universe in an invisible cosmic web that connects galaxies and clusters of galaxies, and feeds them with hydrogen gas.
We don't know what dark matter actually is. Any new evidence that allows us to map its distribution and affect the space around it, is another step in our quest to solve this mystery.
Griffiths stated that while we know it is some kind of matter, we don't know what the constituent particle might be.
We don't know what it does. It has mass and is subjected to gravity. It is important that we have some clues about the particle's size by observing the limits of smoothness or clumping. The more massive the particles, the smaller the dark matter clumps.
The research was published in the Monthly Notices of the Royal Astronomical Society.