Updated
at 2:34 pm EDT
Astronomers
have discovered an enormous, ghostly ring of dark matter 5 billion light-years
away--the most blatant evidence to date for the existence of a mysterious
substance hidden throughout the universe.
Dark matter
makes up a vast majority of gravity-exerting mass in the universe, while only
about 10 percent is matter we can see and touch. If dark
matter didn't exist, scientists say, galaxies like the Milky Way would have
already flown apart from a severe lack of gravitational "glue."
Researchers
pointed the aging but powerful Hubble Space Telescope
toward a cluster of galaxies known as cluster ZwCl0024+1652. At first glance,
the then-unknown ring looked
like a ripple in a pond over the twinkling galactic cluster.
"I was
annoyed when I saw the ring because I thought it was an artifact," said Myungkook
James Jee of Johns Hopkins University.
But it
wasn't a glitch, astronomers announced at a NASA press conference today.
The more
Jee and others tried to remove the ring by tweaking the data, the more the ring-like
anomaly stuck out like a sore thumb. "It took more than a year to
convince myself that the ring was real," Jee said. "I've looked at a number of
clusters and I haven't seen anything like this."
Because so
much dark matter resides in the ring, astronomers said, it bends the light
around it to create the ripple effect--dark matter's calling card. The complete
findings will be detailed in an upcoming issue of the Astrophysical Journal.
The ring,
2.6-million light-years wide, formed when two huge clusters of galaxies slammed
together in a head-on collision 6 to 7 billion years ago, puffing the
mysterious matter outward, the astronomers figure. If the galactic hit-and-run
had occurred outside of Earth's line-of-sight, the result might look more like the "Bullet
cluster"--another cosmic impact site that astronomers view as strong
evidence for dark matter.
Richard
Massey, a Caltech astronomer not connected to the study, said that the finding
is extremely important, especially combined with the Bullet cluster evidence.
But he warned that the discovery still faces skepticism from other astronomers.
"A lot of things can go wrong in producing an image," he said, explaining the shape
could be produced within Hubble's camera itself.
Also, he
said, the failure of Hubble's most powerful camera four months ago doesn't
help. "Just as we were getting to the point to learn how to find dark matter,
it breaks," Massey said.
Richard
White, an astronomer with Space Telescope Science Institute in Maryland, said he also was initially skeptical about the ring of dark matter. "But it shows
up in another Hubble camera's data as well," he said. "It's not as clear, but
it's still there. We argue the ring has been seen twice now."
Unlike
other dark
matter discoveries, the ring is the first collection of dark matter that
differs greatly from the distribution of ordinary matter.
In addition
to using gravity to visualize
the dark matter itself, the team also created computer simulations showing
what happens when galaxy clusters collide. When the two clusters smashed
together, they think, dark matter fell to the center and then sloshed outward.
As it did so, gravity eventually slowed it down and condensed it into a large
ring detected by astronomers.
"By
studying this collision, we are seeing how dark matter responds to
gravity," said Holland Ford, another Johns Hopkins astronomer on the team.
"Nature is doing an experiment for us that we can't do in a lab, and it
agrees with our theoretical models."
Finding dark
matter is not easy because it doesn't shine or reflect light. So astronomers rely
on gravity, which can bend the
light of distant stars when enough mass is present, much like a lens
distorts an image behind it. Thanks to the laws of physics, knowing how much
light is bent tells astronomers how much mass is there. By mapping the gravity's
"footprint," astronomers were able to create a picture of how the dark matter ring is
distributed in the cluster.
In the
image of the cluster, Jee said, "the background galaxies behind the ring show
coherent changes in their shapes due to the presence of the dense ring. It's
like looking at the pebbles on the bottom of a pond with ripples on the surface."
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