Staring into the universe is like looking into a funhouse mirror. This is because gravity distorts space in space and creates optical illusions.
Many of these optical illusions arise when the light of a distant galaxy is magnified, stretched, and brightened as it passes through a massive galaxy or galaxy cluster in front of it. This phenomenon, called the gravitational lens, produces more, stretched and brighter images of the background galaxy.
This phenomenon allows astronomers to study galaxies so far away that they can only be seen by the effects of gravitational lenses. The challenge is to try to reconstruct the distant galaxies from the odd shapes produced by lens.
But astronomers use Hubble Space Telescope fell over such a strange shape during the analysis of quasars, the flaming nuclei of the active galaxies. They discovered two bright, linear objects that appeared to be mirror images of each other. Another oddball object was nearby.
The features confused astronomers so much that it took them several years to solve the mystery. Using two gravitational lens experts, the researchers determined that the three objects were the distorted images of a distant, undiscovered galaxy. But the biggest surprise was that the linear objects were exact copies of each other, a rare event caused by the precise alignment of the background galaxy and the foreground lens.
Astronomers have seen some pretty weird things scattered across our vast universe, from exploding stars to colliding galaxies. So you would think that when they see a strange object in the sky, they would be able to identify it.
But NASAThe Hubble Space Telescope revealed what appear to be a pair of identical objects that look so strange that it took astronomers several years to determine what they are.
“We were really blunt,” said astronomer Timothy Hamilton of Shawnee State University in Portsmouth, Ohio.
The Oddball objects consist of a pair of galaxy bulges (the central star-filled hub of a galaxy) and at least three nearly parallel split stripes. Hamilton discovered them by accident while using Hubble to study a collection of quasars, the burning nuclei of active galaxies.
After chasing blindfold theories, asking for help from colleagues and doing lots of head scraping, Hamilton and the growing team, led by Richard Griffiths of the University of Hawaii at Hilo, finally put together all clues to solve the mystery.
The linear objects were the stretched images of a gravity-lined distant galaxy more than 11 billion light-years away. And it seemed to be mirror images of each other.
The team discovered that the enormous gravity of an intermediate and uncatalogued foreground cluster of galaxies distorted space, enlarged, illuminated, and stretched the image of a distant galaxy behind it, a phenomenon called the gravitational lens. Although Hubble studies reveal many of these funhouse mirror distortions caused by gravitational lenses, this object was uniquely confusing.
In this case, a precise alignment between a background galaxy and a galaxy cluster in the foreground produces two enlarged copies of the same image of the distant galaxy. This rare phenomenon occurs because the background galaxy extends over a ripple in space. This “ripple” is an area of greatest magnification, caused by gravity in dense amounts of dark matter, the invisible glue that makes up most of the mass of the universe. As light from the distant galaxy passes through the cluster along this ripple, two mirror images are produced along with a third image that can be seen to the side.
Griffiths compares this effect to the bright wavy patterns seen at the bottom of a swimming pool. “Think of the rippled surface of a swimming pool on a sunny day that shows patterns of bright light at the bottom of the pool,” he explained. “These bright patterns on the bottom are due to a similar kind of effect as gravitational lenses. Ripples on the surface act as partial lenses and focus sunlight in bright, squirming patterns on the bottom. ”
In the gravitational lensed distant galaxy, the ripple greatly magnifies the light from the background galaxy passing through the cluster. The ripple acts as an imperfectly curved mirror that generates duplicate copies.
Solving the mystery
But this rare phenomenon was not known when Hamilton discovered the strange linear features in 2013.
As he looked through the quasar images, the snapshot of mirrored images and parallel streaks emerged. Hamilton had never seen anything like it before, and neither had other team members.
“My first thought was that they might be interacting galaxies with occasionally outstretched arms,” Hamilton said. “It did not fit really well, but I did not know what else to think.”
So Hamilton and the team began their quest to solve the mystery with these teasing straight lines, later called Hamilton’s object for his discoverer. They showed the strange image to colleagues at astronomy conferences, which provoked a series of reactions, from cosmic strings to planetary nebulae.
But then Griffiths, who was not a member of the original team, offered the most likely explanation when Hamilton showed him the image at a NASA meeting in 2015. It was an enlarged and distorted image caused by a lens phenomenon similar to those seen in Hubble images of other massive galaxy clusters that amplify images of very distant galaxies. Griffiths confirmed this idea when he learned about a similar linear object in one of Hubble’s deep cluster studies.
However, the researchers still had a problem. They could not identify the lens cluster. Usually, astronomers studying galaxy clusters first see the foreground cluster causing the lens, and then they find magnified images of distant galaxies in the cluster. A search of Sloan Digital Sky Survey images revealed that a cluster of galaxies resided in the same area as the magnified images, but it did not appear in any cataloged study. Nevertheless, the fact that the strange images were at the center of a cluster made it clear to Griffiths that the cluster was producing the lensed images.
The researchers’ next step was to determine if the three lens images were at the same distance, and therefore all the distorted portraits were of the same distant galaxy. Spectroscopic measurements with the Gemini and WM Keck observatories in Hawaii helped the scientists make that confirmation, showing that the lensed images were from a galaxy located more than 11 billion light-years away.
The distant galaxy, based on a reconstruction of the third lens image, appears to be an edge-on-barred spiral with continuous, lumpy star formation.
Approximately at the same time as the spectroscopic observations of Griffiths and students in Hilo, a separate group of researchers in Chicago identified the cluster and measured its distance using Sloan data. The cluster lives more than 7 billion light-years away.
But with very little information about the cluster, Griffiths’ team was still struggling to interpret these unusual lens shapes. “This gravitational lens is very different from most lenses studied before by Hubble, especially in the Hubble Frontier Fields study of clusters,” Griffiths explained. “You do not have to stare at those clusters for long to find many lenses. In this object, this is the only objective we have. And we did not even know about the cluster in the beginning. ”
Mapping the invisible
That was when Griffiths called an expert in gravitational lens theory, Jenny Wagner from the University of Heidelberg in Germany. Wagner had researched similar objects and, together with colleague Nicolas Tessore, now at the University of Manchester in England, developed computer software for the interpretation of unique lenses like this. Their software helped the team figure out how all three lens images came to be. They concluded that the dark matter around the stretched images should be “evenly” distributed in space on small scales.
“It’s amazing that we only need two mirror images to get the scale of how lumpy or not dark matter can be in these positions,” Wagner said. “Here we do not use any lens models. We just take observable 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. This already gives us an idea of how smooth the dark fabric should be in these two positions. ”
This result is important, Griffiths said, because astronomers still do not know what dark matter is, nearly a century after its discovery. “We know it’s some kind of substance, but we have no idea what that ingredient is. So we do not know at all how it behaves. We just know that it has mass and is the object of gravity. The significance of the size limits for clumping or smoothness is that it gives us some clues as to what the particle may be. The smaller the dark matter clumps, the more massive the particles must be. ”
The team paper is shown in the September issue of The monthly announcements from the Royal Astronomical Society.
Reference: “Hamilton’s object – a lumpy galaxy bordering on gravity in a galaxy cluster: limitations of lumping dark matter” by Richard E Griffiths, Mitchell Rudisel, Jenny Wagner, Timothy Hamilton, Po-Chieh Huang and Carolin Villforth, 17 May 2021, The monthly announcements from the Royal Astronomical Society.
DOI: 10.1093 / mnras / stab1375
The Hubble Space Telescope is a project for international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, performs Hubble scientific operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, DC