United States astronomers on Tuesday presented the most solid proof yet of the existence of dark matter, a mysterious substance believed to make up more than a quarter of the universe.
Astronomers using the Hubble Space Telescope spotted a ring of dark matter in a galaxy cluster about five billion light years away from Earth, which measured 2,6-million light years across.
”This is the first time we have detected dark matter as having a unique structure that is different from the gas and galaxies in the cluster,” said James Jee, a member of Nasa’s team of astronomers.
Astronomers have long suspected there must be some substance holding galaxy clusters together, otherwise galaxies would only have the gravity from their visible stars, which would not be enough to keep them from flying apart.
Though invisible, astronomers have inferred dark matter exists by observing how its gravity bends the light of more distant background galaxies.
”Although the invisible matter has been found before in other galaxy clusters, it has never been detected to be so largely separated from the hot gas and the galaxies that make up galaxy clusters,” said Jee, who works at the Johns Hopkins University.
”By seeing a dark-matter structure that is not traced by galaxies and hot gas, we can study how it behaves differently from normal matter,” he said.
US astronomers said they made the discovery accidentally while mapping the distribution of dark matter within the ZwC10024+1652 cluster in August 2006.
As they studied their data the astronomers noticed a ripple in the dark matter, similar to ripples created when a stone is plopped into a pond.
Jee said that at first he was upset when he found the ring because he thought there was a flaw in the team’s data reduction.
He then found research published in 2002 suggesting that the cluster under observation had collided with another cluster one to two billion years ago.
The collision occurred along Earth’s line of sight, and from this perspective the dark-matter structure looks like a ring.
”Nature is doing an experiment for us that we can’t do in a lab, and it agrees with our theoretical models,” said team member Holland Ford, also of Johns Hopkins University.
The galaxy cluster collision ”created a ripple of dark matter, which left distinct footprints in the shapes of the background galaxies”, said Jee.
”It’s like looking at the pebbles on the bottom of a pond with ripples on the surface. The pebbles’ shapes appear to change as the ripples pass over them,” Jee said.
”So, too, the background galaxies behind the ring show coherent changes in their shapes due to the presence of the dense ring.” — AFP
