Galaxies are a lot like fingerprints or snowflakes. There are a lot of them, and they may have a lot of characteristics 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, which looked surprisingly similar, astronomers were taken aback.
The object, now named Hamilton’s Object, was discovered by accident by astronomer Timothy Hamilton of Shawnee State University, from data obtained by the Hubble Space Telescope nearly ten years ago.
Both galaxies appeared to be the same shape and had the same dark streaks almost parallel across the galactic bulge – the central region of the galaxy where most of the stars live.
“We were really puzzled”, Hamilton said. “My first thought was that maybe they were interacting galaxies with arms stretched out by the tides. It wasn’t really going well, but I didn’t know what else to think.”
It is not until 2015 that a more plausible answer will emerge. University of Hawaii astronomer Richard Griffiths, upon seeing Hamilton present his object in a meeting, suggested that the culprit could be a rare phenomenon: the gravitational lens.
It is a phenomenon which results purely from a random alignment of massive objects in space. If a massive object is directly between us and a more distant object, a magnification effect occurs due to the gravitational curvature of spacetime around the nearest object.
Any light that then passes through this space-time follows this curvature and enters our telescopes smeared and distorted to varying degrees – but also often magnified and duplicated.
It made a lot more sense than two identical galaxies, especially when Griffiths found another galaxy duplication (as can be seen in the image below).
However, a huge problem remained: what was causing the gravitational curvature? Griffiths and his team set out to search for sky survey data for an object massive enough to produce the lens effect.
And they found it. Between us and the Hamilton object hides a cluster of galaxies that had only been poorly documented. Usually, these findings go the other way – first the cluster is identified, then astronomers set off to search for lens galaxies behind them.
The team’s work revealed that the Hamilton object is about 11 billion light-years away, and the work of another team found that the cluster is about 7 billion light-years away. light.
The galaxy itself is a barred spiral galaxy with the edge facing us, undergoing lumpy and uneven star formation, the researchers determined. Computer simulations then determined that the three duplicate images could only be created if the distribution of dark matter is smooth on a small scale.
“It’s great that we only need two mirror images to measure how much dark matter may or may not be agglomerated at these positions.” said astronomer Jenny Wagner from the University of Heidelberg in Germany.
“Here we don’t use any lens model. We just take the observables of the multiple images and the fact that they can be transformed into each other. They can be folded into each other by our method. already gives an idea of how smooth dark matter must be at these two positions. “
The two identical side-by-side images were created because they overlap a “ripple” in spacetime – an area of greater magnification created by the gravity of a filament of dark matter. Such filaments They are believed to connect the Universe in a vast invisible cosmic network, joining galaxies and galaxy clusters and supplying them with hydrogen gas.
But we don’t really know what dark matter is, so any new discovery that allows us to map where it is, how it’s distributed, and how it affects the space around it is another drop of evidence that will ultimately help us solve the mystery.
“We know it is a form of matter, but we have no idea what the constituent particle is”, Griffiths explained.
“So we don’t know how he’s behaving at all. We just know that it has mass and that it is subject to gravity. The smaller the clusters of dark matter, the more massive the particles must be. “
The research was published in the Monthly notices from the Royal Astronomical Society.