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https://www.newscientist.com/article/dn11860-ring-of-dark-matter-surrounds-cosmic-collision/
15 May 2007
Ring of dark matter surrounds cosmic collision
By David Shiga
A cloud of dark matter has been detected expanding like a smoke ring from a giant collision between galaxy clusters, a team of astronomers says. If confirmed, the ring could offer new clues to the nature of the mysterious matter.
Dark matter is an enigmatic material that does not emit, absorb or reflect light. It reveals itself only by the way its gravity influences normal matter around it and seems to outweigh the universe’s normal matter by a factor of six.
Now, astronomers have discovered what looks like a ring of dark matter expanding from a cosmic clash involving two massive galaxy clusters. If confirmed, the ring will help astronomers investigate how dark matter behaves when disturbed, perhaps providing hints to its nature.
Myungkook James Jee of Johns Hopkins University in Maryland, US, led the team of researchers that made the discovery. They detected the ring in Hubble Space Telescope (HST) observations of a galaxy cluster called Cl 0024+17, which is 5 billion light years from Earth in the direction of the constellation Pisces.
They carefully observed how matter in the cluster bends the paths of light rays coming from distant background galaxies. This ‘gravitational lensing’ effect allowed them to map out the distribution of matter, including dark matter, in the cluster.
Colossal collision
They found a curious ring-like structure around the cluster’s outskirts. The researchers believe the ring is made of dark matter, since there is no concentration of visible matter at the location of the ring. Most of the cluster’s ordinary matter is thought to be in the form of hot gas, which was mapped by the Chandra X-ray Observatory, and is concentrated at the cluster’s core.
Previous studies have hinted that Cl 0024+17 is not a single galaxy cluster but a pair of clusters that have collided. Simulations show that when such a colossal collision occurs – in this case, it involved almost 200 billion Suns’ worth of matter – the cluster’s dark matter explodes outwards, forming a roughly spherical shell. Watch an animation of the collision and dark matter explosion.
Projected on the sky, such a shell would look like the ring observed in the gravitational lensing study, the researchers say. They estimate that the collision happened sometime between 1 billion and 2 billion years ago.
Modified gravity
Some scientists have previously suggested that modifications to Newton’s law of gravity could account for anomalies cited as evidence for dark matter (see Equinox challenge to Newton’s law). This possibility can be difficult to rule out, because normally dark matter and ordinary matter are so well mixed together that it is difficult to pin down effects due to dark matter alone.
But because the ring in Cl 0024+17 is separate from any concentrations of ordinary matter, it would be difficult to explain without dark matter, the researchers say. “I think it’s the strongest evidence for the existence of dark matter to date,” Jee told New Scientist.
A previous study revealed the separation of dark matter and ordinary matter in the Bullet Cluster of galaxies, but in the case of Cl 0024+17, the separation is even more complete.
Marusa Bradac of the Stanford Linear Accelerator Center in (SLAC) in Menlo Park, California, US, who co-authored the Bullet Cluster study, says if the dark matter ring is real, it will be an important laboratory for studying the properties of dark matter.
Instrumental effects
That is because the ring’s characteristics, such as its exact shape, depend on the properties of dark matter particles, like whether they bounce off one another when they meet, or just pass right through one another.
“It’s a very exciting result, and if it’s real, its going to give us a ton of information about dark matter,” she told New Scientist.
But she says gravitational lensing effects of the kind used in the study are very subtle and could be confused with slight distortions that can arise in observations because of imperfect instruments.
“I’m not saying that I don’t trust the result, but at this point I would just be slightly sceptical,” she says. She acknowledges, however, that the team “did do a very careful analysis” of the data.
Exotic particles
The team itself initially suspected the ring was some kind of illusion introduced by Hubble’s instruments but could not find any such effects. In any case, Jee says, if the ring were an instrumental effect, it should show up in similar Hubble observations obtained of other galaxy clusters. But those observations show no sign of rings.
He says the mere existence of the ring shows that the dark matter particles do not interact strongly with one another through any force other than gravity. That is true for the most popular candidates for dark matter – exotic subatomic particles with names like axions and neutralinos.
But alternative candidates, such as “mirror matter” – a form of matter that interacts readily with itself through a range of forces – would tend to collide more frequently. “If there is any collisionary interaction between dark matter particles, then it can smear out the very subtle shell-like structure,” Jee says.
Journal reference: The Astrophysical Journal (in press)
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